Cleanup dead code (#7409)

* Remove unused files

ci/licenses_golden/licenses_flutter

@@ -214,9 +214,7 @@ FILE: ../../../flutter/fml/unique_object.h
 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.dart
 FILE: ../../../flutter/lib/snapshot/snapshot.h
-FILE: ../../../flutter/lib/snapshot/snapshot_fuchsia.dart
 FILE: ../../../flutter/lib/ui/compositing.dart
 FILE: ../../../flutter/lib/ui/compositing/scene.cc
 FILE: ../../../flutter/lib/ui/compositing/scene.h

lib/snapshot/snapshot.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:async';
-import 'dart:collection';
-import 'dart:convert';
-import 'dart:core';
-import 'dart:developer';
-import 'dart:io';
-import 'dart:isolate';
-import 'dart:math';
-import 'dart:typed_data';
-import 'dart:ui';

lib/snapshot/snapshot_fuchsia.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:async';
-import 'dart:collection';
-import 'dart:convert';
-import 'dart:core';
-import 'dart:developer';
-import 'dart:fuchsia';
-import 'dart:io';
-import 'dart:isolate';
-import 'dart:math';
-import 'dart:mozart.internal';
-import 'dart:typed_data';
-import 'dart:ui';
-import 'dart:zircon';

lib/ui/natives.dart

@@ -73,11 +73,3 @@ int _isolateId;
 Function _getPrintClosure() => _print;
 @pragma('vm:entry-point')
 Function _getScheduleMicrotaskClosure() => _scheduleMicrotask;
-
-// Though the "main" symbol is not included in any of the libraries imported
-// above, the builtin library will be included manually during VM setup. This
-// symbol is only necessary for precompilation. It is marked as a stanalone
-// entry point into the VM. This prevents the precompiler from tree shaking
-// away "main".
-@pragma('vm:entry-point')
-Function _getMainClosure() => main;