Provide a flag for controlling the dart2js optimization level when building for web targets (#101945)

Author: jason-simmons

packages/flutter_tools/lib/src/commands/build_web.dart

@@ -63,7 +63,10 @@ class BuildWebCommand extends BuildSubCommand {
           'The value has to start and end with a slash "/". '
           'For more information: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/base'
     );
-
+    argParser.addOption('dart2js-optimization',
+      help: 'Sets the optimization level used for Dart compilation to JavaScript. '
+          'Valid values range from O0 to O4.'
+    );
   }
 
   @override
@@ -120,6 +123,7 @@ class BuildWebCommand extends BuildSubCommand {
       boolArg('source-maps'),
       boolArg('native-null-assertions'),
       baseHref,
+      stringArg('dart2js-optimization'),
     );
     return FlutterCommandResult.success();
   }

packages/flutter_tools/lib/src/isolated/resident_web_runner.dart

@@ -302,6 +302,7 @@ class ResidentWebRunner extends ResidentRunner {
             true,
             debuggingOptions.nativeNullAssertions,
             null,
+            null,
           );
         }
         await device.device.startApp(
@@ -375,6 +376,7 @@ class ResidentWebRunner extends ResidentRunner {
           true,
           debuggingOptions.nativeNullAssertions,
           kBaseHref,
+          null,
         );
       } on ToolExit {
         return OperationResult(1, 'Failed to recompile application.');

packages/flutter_tools/lib/src/web/compile.dart

@@ -25,6 +25,7 @@ Future<void> buildWeb(
   bool sourceMaps,
   bool nativeNullAssertions,
   String? baseHref,
+  String? dart2jsOptimization,
 ) async {
   final bool hasWebPlugins = (await findPlugins(flutterProject))
     .any((Plugin p) => p.platforms.containsKey(WebPlugin.kConfigKey));
@@ -51,6 +52,8 @@ Future<void> buildWeb(
         kNativeNullAssertions: nativeNullAssertions.toString(),
         if (serviceWorkerStrategy != null)
          kServiceWorkerStrategy: serviceWorkerStrategy,
+        if (dart2jsOptimization != null)
+         kDart2jsOptimization: dart2jsOptimization,
         ...buildInfo.toBuildSystemEnvironment(),
       },
       artifacts: globals.artifacts!,

packages/flutter_tools/test/commands.shard/hermetic/build_web_test.dart

@@ -90,7 +90,7 @@ void main() {
     final BuildCommand buildCommand = BuildCommand();
     final CommandRunner<void> runner = createTestCommandRunner(buildCommand);
     setupFileSystemForEndToEndTest(fileSystem);
-    await runner.run(<String>['build', 'web', '--no-pub', '--dart-define=foo=a']);
+    await runner.run(<String>['build', 'web', '--no-pub', '--dart-define=foo=a', '--dart2js-optimization=O3']);
 
     expect(fileSystem.file(fileSystem.path.join('lib', 'generated_plugin_registrant.dart')).existsSync(), true);
   }, overrides: <Type, Generator>{
@@ -106,6 +106,7 @@ void main() {
         'SourceMaps': 'false',
         'NativeNullAssertions': 'true',
         'ServiceWorkerStrategy': 'offline-first',
+        'Dart2jsOptimization': 'O3',
         'BuildMode': 'release',
         'DartDefines': 'Zm9vPWE=,RkxVVFRFUl9XRUJfQVVUT19ERVRFQ1Q9dHJ1ZQ==',
         'DartObfuscation': 'false',