Do not assume Platform.script is a Dart source file during training. (#12256)

Cf. dart-lang/sdk@74cff6c7df3e8f212c7f41a3a25631e007abc0f7

frontend_server/BUILD.gn

@@ -50,6 +50,7 @@ if (is_fuchsia_host || is_fuchsia) {
     training_args = [
       "--train",
       "--sdk-root=$flutter_patched_sdk",
+      rebase_path(main_dart),
     ]
 
     frontend_server_files =

frontend_server/lib/server.dart

@@ -92,6 +92,11 @@ Future<int> starter(
   }
 
   if (options['train']) {
+    if (!options.rest.isNotEmpty) {
+      throw Exception('Must specify input.dart');
+    }
+
+    final String input = options.rest[0];
     final String sdkRoot = options['sdk-root'];
     final Directory temp = Directory.systemTemp.createTempSync('train_frontend_server');
     try {
@@ -105,7 +110,7 @@ Future<int> starter(
       ]);
       compiler ??= _FlutterFrontendCompiler(output);
 
-      await compiler.compile(Platform.script.toFilePath(), options);
+      await compiler.compile(input, options);
       compiler.acceptLastDelta();
       await compiler.recompileDelta();
       compiler.acceptLastDelta();

shell/platform/fuchsia/dart/BUILD.gn

@@ -15,7 +15,10 @@ application_snapshot("kernel_compiler") {
 
   dot_packages = rebase_path("//third_party/dart/.packages")
 
-  training_args = [ "--train" ]
+  training_args = [
+    "--train",
+    rebase_path(main_dart),
+  ]
 
   kernel_compiler_files =
       exec_script("//third_party/dart/tools/list_dart_files.py",