[flutter_tool] advertise the default value for --dart2js-optimization (#121621)

[flutter_tool] advertise the default value for --dart2js-optimization

packages/flutter_tools/lib/src/build_system/targets/web.dart

@@ -39,6 +39,11 @@ const String kHasWebPlugins = 'HasWebPlugins';
 /// Valid values are O1 (lowest, profile default) to O4 (highest, release default).
 const String kDart2jsOptimization = 'Dart2jsOptimization';
 
+/// The default optimization level for dart2js.
+///
+/// Maps to [kDart2jsOptimization].
+const String kDart2jsDefaultOptimizationLevel = 'O4';
+
 /// If `--dump-info` should be passed to dart2js.
 const String kDart2jsDumpInfo = 'Dart2jsDumpInfo';
 
@@ -235,7 +240,7 @@ class Dart2JSTarget extends Dart2WebTarget {
       throw Exception(_collectOutput(kernelResult));
     }
 
-    final String? dart2jsOptimization = environment.defines[kDart2jsOptimization];
+    final String dart2jsOptimization = environment.defines[kDart2jsOptimization] ?? kDart2jsDefaultOptimizationLevel;
     final bool dumpInfo = environment.defines[kDart2jsDumpInfo] == 'true';
     final bool noFrequencyBasedMinification = environment.defines[kDart2jsNoFrequencyBasedMinification] == 'true';
     final File outputJSFile = environment.buildDir.childFile('main.dart.js');
@@ -243,7 +248,7 @@ class Dart2JSTarget extends Dart2WebTarget {
 
     final ProcessResult javaScriptResult = await environment.processManager.run(<String>[
       ...sharedCommandOptions,
-      if (dart2jsOptimization != null) '-$dart2jsOptimization' else '-O4',
+      '-$dart2jsOptimization',
       if (buildMode == BuildMode.profile) '--no-minify',
       if (dumpInfo) '--dump-info',
       if (noFrequencyBasedMinification) '--no-frequency-based-minification',

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

@@ -73,7 +73,8 @@ class BuildWebCommand extends BuildSubCommand {
     );
     argParser.addOption('dart2js-optimization',
       help: 'Sets the optimization level used for Dart compilation to JavaScript. '
-          'Valid values range from O0 to O4.'
+          'Valid values range from O0 to O4.',
+          defaultsTo: kDart2jsDefaultOptimizationLevel
     );
     argParser.addFlag('dump-info', negatable: false,
       help: 'Passes "--dump-info" to the Javascript compiler which generates '
@@ -147,7 +148,7 @@ class BuildWebCommand extends BuildSubCommand {
       boolArgDeprecated('native-null-assertions'),
       boolArgDeprecated('wasm'),
       baseHref: baseHref,
-      dart2jsOptimization: stringArgDeprecated('dart2js-optimization'),
+      dart2jsOptimization: stringArgDeprecated('dart2js-optimization') ?? kDart2jsDefaultOptimizationLevel,
       outputDirectoryPath: outputDirectoryPath,
       dumpInfo: boolArgDeprecated('dump-info'),
       noFrequencyBasedMinification: boolArgDeprecated('no-frequency-based-minification'),

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

@@ -18,6 +18,8 @@ import '../plugins.dart';
 import '../project.dart';
 import 'migrations/scrub_generated_plugin_registrant.dart';
 
+export '../build_system/targets/web.dart' show kDart2jsDefaultOptimizationLevel;
+
 Future<void> buildWeb(
   FlutterProject flutterProject,
   String target,
@@ -27,7 +29,7 @@ Future<void> buildWeb(
   bool sourceMaps,
   bool nativeNullAssertions,
   bool isWasm, {
-  String? dart2jsOptimization,
+  String dart2jsOptimization = kDart2jsDefaultOptimizationLevel,
   String? baseHref,
   bool dumpInfo = false,
   bool noFrequencyBasedMinification = false,
@@ -68,7 +70,6 @@ Future<void> buildWeb(
           kSourceMapsEnabled: sourceMaps.toString(),
           kNativeNullAssertions: nativeNullAssertions.toString(),
           kServiceWorkerStrategy: serviceWorkerStrategy,
-          if (dart2jsOptimization != null)
           kDart2jsOptimization: dart2jsOptimization,
           kDart2jsDumpInfo: dumpInfo.toString(),
           kDart2jsNoFrequencyBasedMinification: noFrequencyBasedMinification.toString(),

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

@@ -183,6 +183,7 @@ void main() {
         'ServiceWorkerStrategy': 'offline-first',
         'Dart2jsDumpInfo': 'false',
         'Dart2jsNoFrequencyBasedMinification': 'false',
+        'Dart2jsOptimization': 'O4',
         'BuildMode': 'release',
         'DartDefines': 'RkxVVFRFUl9XRUJfQVVUT19ERVRFQ1Q9dHJ1ZQ==',
         'DartObfuscation': 'false',