Teach frontend compiler to replace `toString` with `super.toString` for selected packages (#17068)

Adds annotation `keepToString` to opt out.

analysis_options.yaml

@@ -10,9 +10,13 @@
 #    private fields, especially on the Window object):
 
 analyzer:
-  # this test pretends to be part of dart:ui and results in lots of false
-  # positives.
-  exclude: [ testing/dart/window_hooks_integration_test.dart ]
+  exclude: [
+    # this test pretends to be part of dart:ui and results in lots of false
+    # positives.
+    testing/dart/window_hooks_integration_test.dart,
+    # Fixture depends on dart:ui and raises false positives.
+    flutter_frontend_server/test/fixtures/lib/main.dart
+  ]
   strong-mode:
     implicit-casts: false
     implicit-dynamic: false

ci/licenses_golden/licenses_flutter

@@ -269,6 +269,7 @@ FILE: ../../../flutter/lib/io/dart_io.cc
 FILE: ../../../flutter/lib/io/dart_io.h
 FILE: ../../../flutter/lib/snapshot/libraries.json
 FILE: ../../../flutter/lib/snapshot/snapshot.h
+FILE: ../../../flutter/lib/ui/annotations.dart
 FILE: ../../../flutter/lib/ui/channel_buffers.dart
 FILE: ../../../flutter/lib/ui/compositing.dart
 FILE: ../../../flutter/lib/ui/compositing/scene.cc
@@ -494,6 +495,7 @@ FILE: ../../../flutter/lib/web_ui/lib/src/engine/util.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/validators.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/vector_math.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/window.dart
+FILE: ../../../flutter/lib/web_ui/lib/src/ui/annotations.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/ui/canvas.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/ui/channel_buffers.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/ui/compositing.dart

flutter_frontend_server/lib/server.dart

@@ -8,9 +8,6 @@ import 'dart:async';
 import 'dart:io' hide FileSystemEntity;
 
 import 'package:args/args.dart';
-import 'package:path/path.dart' as path;
-
-import 'package:vm/incremental_compiler.dart';
 import 'package:frontend_server/frontend_server.dart' as frontend
     show
         FrontendCompiler,
@@ -19,6 +16,9 @@ import 'package:frontend_server/frontend_server.dart' as frontend
         argParser,
         usage,
         ProgramTransformer;
+import 'package:kernel/ast.dart';
+import 'package:path/path.dart' as path;
+import 'package:vm/incremental_compiler.dart';
 
 /// Wrapper around [FrontendCompiler] that adds [widgetCreatorTracker] kernel
 /// transformation to the compilation.
@@ -107,6 +107,13 @@ Future<int> starter(
   frontend.ProgramTransformer transformer,
 }) async {
   ArgResults options;
+  frontend.argParser.addMultiOption(
+    'delete-tostring-package-uri',
+    help: 'Replaces implementations of `toString` with `super.toString()` for '
+          'specified package',
+    valueHelp: 'dart:ui',
+    defaultsTo: const <String>[],
+  );
   try {
     options = frontend.argParser.parse(args);
   } catch (error) {
@@ -115,6 +122,8 @@ Future<int> starter(
     return 1;
   }
 
+  final Set<String> deleteToStringPackageUris = (options['delete-tostring-package-uri'] as List<String>).toSet();
+
   if (options['train'] as bool) {
     if (!options.rest.isNotEmpty) {
       throw Exception('Must specify input.dart');
@@ -137,7 +146,10 @@ Future<int> starter(
           '--gen-bytecode',
           '--bytecode-options=source-positions,local-var-info,debugger-stops,instance-field-initializers,keep-unreachable-code,avoid-closure-call-instructions',
         ]);
-        compiler ??= _FlutterFrontendCompiler(output);
+        compiler ??= _FlutterFrontendCompiler(
+          output,
+          transformer: ToStringTransformer(null, deleteToStringPackageUris),
+        );
 
         await compiler.compile(input, options);
         compiler.acceptLastDelta();
@@ -156,7 +168,7 @@ Future<int> starter(
   }
 
   compiler ??= _FlutterFrontendCompiler(output,
-      transformer: transformer,
+      transformer: ToStringTransformer(transformer, deleteToStringPackageUris),
       useDebuggerModuleNames: options['debugger-module-names'] as bool,
       unsafePackageSerialization:
           options['unsafe-package-serialization'] as bool);
@@ -169,3 +181,85 @@ Future<int> starter(
   frontend.listenAndCompile(compiler, input ?? stdin, options, completer);
   return completer.future;
 }
+
+// Transformer/visitor for toString
+// If we add any more of these, they really should go into a separate library.
+
+/// A [RecursiveVisitor] that replaces [Object.toString] overrides with
+/// `super.toString()`.
+class ToStringVisitor extends RecursiveVisitor<void> {
+  /// The [packageUris] must not be null.
+  ToStringVisitor(this._packageUris) : assert(_packageUris != null);
+
+  /// A set of package URIs to apply this transformer to, e.g. 'dart:ui' and
+  /// 'package:flutter/foundation.dart'.
+  final Set<String> _packageUris;
+
+  /// Turn 'dart:ui' into 'dart:ui', or
+  /// 'package:flutter/src/semantics_event.dart' into 'package:flutter'.
+  String _importUriToPackage(Uri importUri) => '${importUri.scheme}:${importUri.pathSegments.first}';
+
+  bool _isInTargetPackage(Procedure node) {
+    return _packageUris.contains(_importUriToPackage(node.enclosingLibrary.importUri));
+  }
+
+  bool _hasKeepAnnotation(Procedure node) {
+    for (ConstantExpression expression in node.annotations.whereType<ConstantExpression>()) {
+      if (expression.constant is! InstanceConstant) {
+        continue;
+      }
+      final InstanceConstant constant = expression.constant as InstanceConstant;
+      if (constant.classNode.name == '_KeepToString' && constant.classNode.enclosingLibrary.importUri.toString() == 'dart:ui') {
+        return true;
+      }
+    }
+    return false;
+  }
+
+  @override
+  void visitProcedure(Procedure node) {
+    if (
+      node.name.name        == 'toString' &&
+      node.enclosingClass   != null       &&
+      node.enclosingLibrary != null       &&
+      !node.isStatic                      &&
+      !node.isAbstract                    &&
+      _isInTargetPackage(node)            &&
+      !_hasKeepAnnotation(node)
+    ) {
+      node.function.body.replaceWith(
+        ReturnStatement(
+          SuperMethodInvocation(
+            node.name,
+            Arguments(<Expression>[]),
+          ),
+        ),
+      );
+    }
+  }
+
+  @override
+  void defaultMember(Member node) {}
+}
+
+/// Replaces [Object.toString] overrides with calls to super for the specified
+/// [packageUris].
+class ToStringTransformer extends frontend.ProgramTransformer {
+  /// The [packageUris] parameter must not be null, but may be empty.
+  ToStringTransformer(this._child, this._packageUris) : assert(_packageUris != null);
+
+  final frontend.ProgramTransformer _child;
+
+  /// A set of package URIs to apply this transformer to, e.g. 'dart:ui' and
+  /// 'package:flutter/foundation.dart'.
+  final Set<String> _packageUris;
+
+  @override
+  void transform(Component component) {
+    assert(_child is! ToStringTransformer);
+    if (_packageUris.isNotEmpty) {
+      component.visitChildren(ToStringVisitor(_packageUris));
+    }
+    _child?.transform(component);
+  }
+}

flutter_frontend_server/test/fixtures/.gitignore

+!.packages

flutter_frontend_server/test/fixtures/.packages

+# Generated by pub on 2020-01-15 10:08:29.776333.
+flutter_frontend_fixtures:lib/

flutter_frontend_server/test/fixtures/lib/main.dart

+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:convert';
+import 'dart:ui';
+
+void main() {
+  final Paint paint = Paint()..color = Color(0xFFFFFFFF);
+  print(jsonEncode(<String, String>{
+    'Paint.toString': paint.toString(),
+    'Foo.toString': Foo().toString(),
+    'Keep.toString': Keep().toString(),
+  }));
+}
+
+class Foo {
+  @override
+  String toString() => 'I am a Foo';
+}
+
+class Keep {
+  @keepToString
+  @override
+  String toString() => 'I am a Keep';
+}

flutter_frontend_server/test/to_string_test.dart

+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:io';
+
+import 'package:flutter_frontend_server/server.dart';
+import 'package:frontend_server/frontend_server.dart' as frontend show ProgramTransformer;
+import 'package:kernel/kernel.dart';
+import 'package:mockito/mockito.dart';
+import 'package:path/path.dart' as path;
+
+import 'package:test/test.dart';
+
+void main(List<String> args) async {
+  if (args.length != 2) {
+    stderr.writeln('The first argument must be the path to the forntend server dill.');
+    stderr.writeln('The second argument must be the path to the flutter_patched_sdk');
+    exit(-1);
+  }
+
+  const Set<String> uiAndFlutter = <String>{
+    'dart:ui',
+    'package:flutter',
+  };
+
+  test('No packages', () {
+    final ToStringTransformer transformer = ToStringTransformer(null, <String>{});
+
+    final MockComponent component = MockComponent();
+    transformer.transform(component);
+    verifyNever(component.visitChildren(any));
+  });
+
+  test('dart:ui package', () {
+    final ToStringTransformer transformer = ToStringTransformer(null, uiAndFlutter);
+
+    final MockComponent component = MockComponent();
+    transformer.transform(component);
+    verify(component.visitChildren(any)).called(1);
+  });
+
+  test('Child transformer', () {
+    final MockTransformer childTransformer = MockTransformer();
+    final ToStringTransformer transformer = ToStringTransformer(childTransformer, <String>{});
+
+    final MockComponent component = MockComponent();
+    transformer.transform(component);
+    verifyNever(component.visitChildren(any));
+    verify(childTransformer.transform(component)).called(1);
+  });
+
+  test('ToStringVisitor ignores non-toString procedures', () {
+    final ToStringVisitor visitor = ToStringVisitor(uiAndFlutter);
+    final MockProcedure procedure = MockProcedure();
+    when(procedure.name).thenReturn(Name('main'));
+    when(procedure.annotations).thenReturn(const <Expression>[]);
+
+    visitor.visitProcedure(procedure);
+    verifyNever(procedure.enclosingLibrary);
+  });
+
+  test('ToStringVisitor ignores top level toString', () {
+    // i.e. a `toString` method specified at the top of a library, like:
+    //
+    // void main() {}
+    // String toString() => 'why?';
+    final ToStringVisitor visitor = ToStringVisitor(uiAndFlutter);
+    final MockProcedure procedure = MockProcedure();
+    final MockFunctionNode function = MockFunctionNode();
+    final MockStatement statement = MockStatement();
+    final Library library = Library(Uri.parse('package:some_package/src/blah.dart'));
+    when(procedure.function).thenReturn(function);
+    when(procedure.name).thenReturn(Name('toString'));
+    when(procedure.annotations).thenReturn(const <Expression>[]);
+    when(procedure.enclosingLibrary).thenReturn(library);
+    when(procedure.enclosingClass).thenReturn(null);
+    when(procedure.isAbstract).thenReturn(false);
+    when(procedure.isStatic).thenReturn(false);
+    when(function.body).thenReturn(statement);
+
+    visitor.visitProcedure(procedure);
+    verifyNever(statement.replaceWith(any));
+  });
+
+  test('ToStringVisitor ignores abstract toString', () {
+    final ToStringVisitor visitor = ToStringVisitor(uiAndFlutter);
+    final MockProcedure procedure = MockProcedure();
+    final MockFunctionNode function = MockFunctionNode();
+    final MockStatement statement = MockStatement();
+    final Library library = Library(Uri.parse('package:some_package/src/blah.dart'));
+    when(procedure.function).thenReturn(function);
+    when(procedure.name).thenReturn(Name('toString'));
+    when(procedure.annotations).thenReturn(const <Expression>[]);
+    when(procedure.enclosingLibrary).thenReturn(library);
+    when(procedure.enclosingClass).thenReturn(Class());
+    when(procedure.isAbstract).thenReturn(true);
+    when(procedure.isStatic).thenReturn(false);
+    when(function.body).thenReturn(statement);
+
+    visitor.visitProcedure(procedure);
+    verifyNever(statement.replaceWith(any));
+  });
+
+  test('ToStringVisitor ignores static toString', () {
+    final ToStringVisitor visitor = ToStringVisitor(uiAndFlutter);
+    final MockProcedure procedure = MockProcedure();
+    final MockFunctionNode function = MockFunctionNode();
+    final MockStatement statement = MockStatement();
+    final Library library = Library(Uri.parse('package:some_package/src/blah.dart'));
+    when(procedure.function).thenReturn(function);
+    when(procedure.name).thenReturn(Name('toString'));
+    when(procedure.annotations).thenReturn(const <Expression>[]);
+    when(procedure.enclosingLibrary).thenReturn(library);
+    when(procedure.enclosingClass).thenReturn(Class());
+    when(procedure.isAbstract).thenReturn(false);
+    when(procedure.isStatic).thenReturn(true);
+    when(function.body).thenReturn(statement);
+
+    visitor.visitProcedure(procedure);
+    verifyNever(statement.replaceWith(any));
+  });
+
+  test('ToStringVisitor ignores non-specified libraries', () {
+    final ToStringVisitor visitor = ToStringVisitor(uiAndFlutter);
+    final MockProcedure procedure = MockProcedure();
+    final MockFunctionNode function = MockFunctionNode();
+    final MockStatement statement = MockStatement();
+    final Library library = Library(Uri.parse('package:some_package/src/blah.dart'));
+    when(procedure.function).thenReturn(function);
+    when(procedure.name).thenReturn(Name('toString'));
+    when(procedure.annotations).thenReturn(const <Expression>[]);
+    when(procedure.enclosingLibrary).thenReturn(library);
+    when(procedure.enclosingClass).thenReturn(Class());
+    when(procedure.isAbstract).thenReturn(false);
+    when(procedure.isStatic).thenReturn(false);
+    when(function.body).thenReturn(statement);
+
+    visitor.visitProcedure(procedure);
+    verifyNever(statement.replaceWith(any));
+  });
+
+  test('ToStringVisitor ignores @keepToString', () {
+    final ToStringVisitor visitor = ToStringVisitor(uiAndFlutter);
+    final MockProcedure procedure = MockProcedure();
+    final MockFunctionNode function = MockFunctionNode();
+    final MockStatement statement = MockStatement();
+    final Library library = Library(Uri.parse('dart:ui'));
+    final Name name = Name('toString');
+    final Class annotation = Class(name: '_KeepToString')..parent = Library(Uri.parse('dart:ui'));
+
+    when(procedure.function).thenReturn(function);
+    when(procedure.name).thenReturn(name);
+    when(procedure.annotations).thenReturn(<Expression>[
+      ConstantExpression(
+        InstanceConstant(
+          Reference()..node = annotation,
+          <DartType>[],
+          <Reference, Constant>{},
+        ),
+      ),
+    ]);
+
+    when(procedure.enclosingLibrary).thenReturn(library);
+    when(procedure.enclosingClass).thenReturn(Class());
+    when(procedure.isAbstract).thenReturn(false);
+    when(procedure.isStatic).thenReturn(false);
+    when(function.body).thenReturn(statement);
+
+    visitor.visitProcedure(procedure);
+    verifyNever(statement.replaceWith(any));
+  });
+
+  void _validateReplacement(MockStatement body) {
+    final ReturnStatement replacement = verify(body.replaceWith(captureAny)).captured.single as ReturnStatement;
+    expect(replacement.expression, isA<SuperMethodInvocation>());
+    final SuperMethodInvocation superMethodInvocation = replacement.expression as SuperMethodInvocation;
+    expect(superMethodInvocation.name.name, 'toString');
+  }
+
+  test('ToStringVisitor replaces toString in specified libraries (dart:ui)', () {
+    final ToStringVisitor visitor = ToStringVisitor(uiAndFlutter);
+    final MockProcedure procedure = MockProcedure();
+    final MockFunctionNode function = MockFunctionNode();
+    final MockStatement statement = MockStatement();
+    final Library library = Library(Uri.parse('dart:ui'));
+    final Name name = Name('toString');
+
+    when(procedure.function).thenReturn(function);
+    when(procedure.name).thenReturn(name);
+    when(procedure.annotations).thenReturn(const <Expression>[]);
+    when(procedure.enclosingLibrary).thenReturn(library);
+    when(procedure.enclosingClass).thenReturn(Class());
+    when(procedure.isAbstract).thenReturn(false);
+    when(procedure.isStatic).thenReturn(false);
+    when(function.body).thenReturn(statement);
+
+    visitor.visitProcedure(procedure);
+    _validateReplacement(statement);
+  });
+
+  test('ToStringVisitor replaces toString in specified libraries (package:flutter)', () {
+    final ToStringVisitor visitor = ToStringVisitor(uiAndFlutter);
+    final MockProcedure procedure = MockProcedure();
+    final MockFunctionNode function = MockFunctionNode();
+    final MockStatement statement = MockStatement();
+    final Library library = Library(Uri.parse('package:flutter/src/foundation.dart'));
+    final Name name = Name('toString');
+
+    when(procedure.function).thenReturn(function);
+    when(procedure.name).thenReturn(name);
+    when(procedure.annotations).thenReturn(const <Expression>[]);
+    when(procedure.enclosingLibrary).thenReturn(library);
+    when(procedure.enclosingClass).thenReturn(Class());
+    when(procedure.isAbstract).thenReturn(false);
+    when(procedure.isStatic).thenReturn(false);
+    when(function.body).thenReturn(statement);
+
+    visitor.visitProcedure(procedure);
+    _validateReplacement(statement);
+  });
+
+  group('Integration tests',  () {
+    final String dart = Platform.resolvedExecutable;
+    final String frontendServer = args[0];
+    final String sdkRoot = args[1];
+    final String basePath = path.canonicalize(path.join(path.dirname(Platform.script.path), '..'));
+    final String fixtures = path.join(basePath, 'test', 'fixtures');
+    final String mainDart = path.join(fixtures, 'lib', 'main.dart');
+    final String dotPackages = path.join(fixtures, '.packages');
+    final String regularDill = path.join(fixtures, 'toString.dill');
+    final String transformedDill = path.join(fixtures, 'toStringTransformed.dill');
+
+
+    void _checkProcessResult(ProcessResult result) {
+      if (result.exitCode != 0) {
+        stdout.writeln(result.stdout);
+        stderr.writeln(result.stderr);
+      }
+      expect(result.exitCode, 0);
+    }
+
+    test('Without flag', () async {
+      _checkProcessResult(Process.runSync(dart, <String>[
+        frontendServer,
+        '--sdk-root=$sdkRoot',
+        '--target=flutter',
+        '--packages=$dotPackages',
+        '--output-dill=$regularDill',
+        mainDart,
+      ]));
+      final ProcessResult runResult = Process.runSync(dart, <String>[regularDill]);
+      _checkProcessResult(runResult);
+      expect(
+        runResult.stdout.trim(),
+        '{"Paint.toString":"Paint(Color(0xffffffff))",'
+         '"Foo.toString":"I am a Foo",'
+         '"Keep.toString":"I am a Keep"}',
+      );
+    });
+
+    test('With flag', () async {
+      _checkProcessResult(Process.runSync(dart, <String>[
+        frontendServer,
+        '--sdk-root=$sdkRoot',
+        '--target=flutter',
+        '--packages=$dotPackages',
+        '--output-dill=$transformedDill',
+        '--delete-tostring-package-uri', 'dart:ui',
+        '--delete-tostring-package-uri', 'package:flutter_frontend_fixtures',
+        mainDart,
+      ]));
+      final ProcessResult runResult = Process.runSync(dart, <String>[transformedDill]);
+      _checkProcessResult(runResult);
+      expect(
+        runResult.stdout.trim(),
+        '{"Paint.toString":"Instance of \'Paint\'",'
+         '"Foo.toString":"Instance of \'Foo\'",'
+         '"Keep.toString":"I am a Keep"}',
+      );
+    });
+  });
+}
+
+class MockComponent extends Mock implements Component {}
+class MockTransformer extends Mock implements frontend.ProgramTransformer {}
+class MockProcedure extends Mock implements Procedure {}
+class MockFunctionNode extends Mock implements FunctionNode {}
+class MockStatement extends Mock implements Statement {}

lib/ui/annotations.dart

+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// TODO(dnfield): Remove unused_import ignores when https://github.com/dart-lang/sdk/issues/35164 is resolved.
+
+// @dart = 2.6
+part of dart.ui;
+
+// TODO(dnfield): Update this if/when we default this to on in the tool,
+// see: https://github.com/flutter/flutter/issues/52759
+/// Annotation used by Flutter's Dart compiler to indicate that an
+/// [Object.toString] override should not be replaced with a supercall.
+///
+/// Since `dart:ui` and `package:flutter` override `toString` purely for
+/// debugging purposes, the frontend compiler is instructed to replace all
+/// `toString` bodies with `return super.toString()` during compilation. This
+/// significantly reduces release code size, and would make it impossible to
+/// implement a meaningful override of `toString` for release mode without
+/// disabling the feature and losing the size savings. If a package uses this
+/// feature and has some unavoidable need to keep the `toString` implementation
+/// for a specific class, applying this annotation will direct the compiler
+/// to leave the method body as-is.
+///
+/// For example, in the following class the `toString` method will remain as
+/// `return _buffer.toString();`, even if the  `--delete-tostring-package-uri`
+/// option would otherwise apply and replace it with `return super.toString()`.
+///
+/// ```dart
+/// class MyStringBuffer {
+///   StringBuffer _buffer = StringBuffer();
+///
+///   // ...
+///
+///   @keepToString
+///   @override
+///   String toString() {
+///     return _buffer.toString();
+///   }
+/// }
+/// ```
+const _KeepToString keepToString = _KeepToString();
+
+class _KeepToString {
+  const _KeepToString();
+}

lib/ui/dart_ui.gni

@@ -3,6 +3,7 @@
 # found in the LICENSE file.
 
 dart_ui_files = [
+  "//flutter/lib/ui/annotations.dart",
   "//flutter/lib/ui/channel_buffers.dart",
   "//flutter/lib/ui/compositing.dart",
   "//flutter/lib/ui/geometry.dart",

lib/ui/ui.dart

@@ -23,6 +23,7 @@ import 'dart:math' as math;
 import 'dart:nativewrappers';
 import 'dart:typed_data';
 
+part 'annotations.dart';
 part 'channel_buffers.dart';
 part 'compositing.dart';
 part 'geometry.dart';

lib/web_ui/lib/src/ui/annotations.dart

+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// TODO(dnfield): Remove unused_import ignores when https://github.com/dart-lang/sdk/issues/35164 is resolved.
+
+// @dart = 2.6
+part of ui;
+
+// TODO(dnfield): Update this if/when we default this to on in the tool,
+// see: https://github.com/flutter/flutter/issues/52759
+/// Annotation used by Flutter's Dart compiler to indicate that an
+/// [Object.toString] override should not be replaced with a supercall.
+///
+/// Since `dart:ui` and `package:flutter` override `toString` purely for
+/// debugging purposes, the frontend compiler is instructed to replace all
+/// `toString` bodies with `return super.toString()` during compilation. This
+/// significantly reduces release code size, and would make it impossible to
+/// implement a meaningful override of `toString` for release mode without
+/// disabling the feature and losing the size savings. If a package uses this
+/// feature and has some unavoidable need to keep the `toString` implementation
+/// for a specific class, applying this annotation will direct the compiler
+/// to leave the method body as-is.
+///
+/// For example, in the following class the `toString` method will remain as
+/// `return _buffer.toString();`, even if the  `--delete-tostring-package-uri`
+/// option would otherwise apply and replace it with `return super.toString()`.
+///
+/// ```dart
+/// class MyStringBuffer {
+///   StringBuffer _buffer = StringBuffer();
+///
+///   // ...
+///
+///   @keepToString
+///   @override
+///   String toString() {
+///     return _buffer.toString();
+///   }
+/// }
+/// ```
+const _KeepToString keepToString = _KeepToString();
+
+class _KeepToString {
+  const _KeepToString();
+}

lib/web_ui/lib/ui.dart

@@ -24,6 +24,7 @@ export 'src/engine.dart'
         webOnlySetPluginHandler,
         webOnlyInitializeEngine;
 
+part 'src/ui/annotations.dart';
 part 'src/ui/canvas.dart';
 part 'src/ui/channel_buffers.dart';
 part 'src/ui/compositing.dart';

testing/run_tests.py

@@ -330,6 +330,23 @@ def RunDartTests(build_dir, filter, verbose_dart_snapshot):
       RunDartTest(build_dir, dart_test_file, verbose_dart_snapshot, True)
       RunDartTest(build_dir, dart_test_file, verbose_dart_snapshot, False)
 
+
+def RunFrontEndServerTests(build_dir):
+  test_dir = os.path.join(buildroot_dir, 'flutter', 'flutter_frontend_server')
+  dart_tests = glob.glob('%s/test/*_test.dart' % test_dir)
+  for dart_test_file in dart_tests:
+    opts = [
+      dart_test_file,
+      os.path.join(build_dir, 'gen', 'frontend_server.dart.snapshot'),
+      os.path.join(build_dir, 'flutter_patched_sdk')]
+    RunEngineExecutable(
+      build_dir,
+      os.path.join('dart-sdk', 'bin', 'dart'),
+      None,
+      flags=opts,
+      cwd=test_dir)
+
+
 def RunConstFinderTests(build_dir):
   test_dir = os.path.join(buildroot_dir, 'flutter', 'tools', 'const_finder', 'test')
   opts = [
@@ -376,6 +393,7 @@ def main():
     dart_filter = args.dart_filter.split(',') if args.dart_filter else None
     RunDartTests(build_dir, dart_filter, args.verbose_dart_snapshot)
     RunConstFinderTests(build_dir)
+    RunFrontEndServerTests(build_dir)
 
   if 'java' in types:
     assert not IsWindows(), "Android engine files can't be compiled on Windows."