Add support for flutter build web --debug. (#157186)

There have been various requests for this for a while. See https://github.com/flutter/flutter/issues/96283 for an example. This has become more important with dart2wasm builds in the mix, as the profiling versions of the dart2wasm builds are a lot less debuggable than the debug versions. Most of this is already wired up so it just requires taking out a few explicit checks for it and making sure that we compile with the right optimization levels when compiling debug.

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

@@ -28,7 +28,7 @@ class BuildWebCommand extends BuildSubCommand {
     usesPubOption();
     usesBuildNumberOption();
     usesBuildNameOption();
-    addBuildModeFlags(verboseHelp: verboseHelp, excludeDebug: true);
+    addBuildModeFlags(verboseHelp: verboseHelp);
     usesDartDefineOption();
     addEnableExperimentation(hide: !verboseHelp);
     addNullSafetyModeOptions(hide: !verboseHelp);
@@ -62,7 +62,6 @@ class BuildWebCommand extends BuildSubCommand {
       abbr: 'O',
       help:
           'Sets the optimization level used for Dart compilation to JavaScript/Wasm.',
-      defaultsTo: '${WebCompilerConfig.kDefaultOptimizationLevel}',
       allowed: const <String>['0', '1', '2', '3', '4'],
     );
     argParser.addFlag(
@@ -134,10 +133,11 @@ class BuildWebCommand extends BuildSubCommand {
       throwToolExit('"build web" is not currently supported. To enable, run "flutter config --enable-web".');
     }
 
-    final int optimizationLevel = int.parse(stringArg('optimization-level')!);
+    final String? optimizationLevelArg = stringArg('optimization-level');
+    final int? optimizationLevel = optimizationLevelArg != null ? int.parse(optimizationLevelArg) : null;
 
     final String? dart2jsOptimizationLevelValue = stringArg('dart2js-optimization');
-    final int jsOptimizationLevel =  dart2jsOptimizationLevelValue != null
+    final int? jsOptimizationLevel =  dart2jsOptimizationLevelValue != null
         ? int.parse(dart2jsOptimizationLevelValue.substring(1))
         : optimizationLevel;
 
@@ -191,9 +191,6 @@ class BuildWebCommand extends BuildSubCommand {
 
     final String target = stringArg('target')!;
     final BuildInfo buildInfo = await getBuildInfo();
-    if (buildInfo.isDebug) {
-      throwToolExit('debug builds cannot be built directly for the web. Try using "flutter run"');
-    }
     final String? baseHref = stringArg('base-href');
     if (baseHref != null && !(baseHref.startsWith('/') && baseHref.endsWith('/'))) {
       throwToolExit(

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

@@ -13,22 +13,29 @@ enum CompileTarget {
 
 sealed class WebCompilerConfig {
   const WebCompilerConfig({required this.renderer,
-                           required this.optimizationLevel,
+                           this.optimizationLevel,
                            required this.sourceMaps});
-
-  /// The default optimization level for dart2js/dart2wasm.
-  static const int kDefaultOptimizationLevel = 4;
-
   /// Build environment flag for [optimizationLevel].
   static const String kOptimizationLevel = 'OptimizationLevel';
 
   /// Build environment flag for [sourceMaps].
   static const String kSourceMapsEnabled = 'SourceMaps';
 
-  /// The compiler optimization level.
+  /// Calculates the optimization level for dart2js/dart2wasm for the given
+  /// build mode.
+  int optimizationLevelForBuildMode(BuildMode mode) =>
+    optimizationLevel ?? switch (mode) {
+      BuildMode.debug => 0,
+      BuildMode.profile || BuildMode.release => 4,
+      BuildMode.jitRelease => throw ArgumentError('Invalid build mode for web'),
+    };
+
+  /// The compiler optimization level specified by the user.
   ///
   /// Valid values are O0 (lowest, debug default) to O4 (highest, release default).
-  final int optimizationLevel;
+  /// If the value is null, the user hasn't specified an optimization level and an
+  /// appropriate default for the build mode will be used instead.
+  final int? optimizationLevel;
 
   /// `true` if the compiler build should output source maps.
   final bool sourceMaps;
@@ -40,7 +47,7 @@ sealed class WebCompilerConfig {
   String get buildKey;
 
   Map<String, Object> get buildEventAnalyticsValues => <String, Object>{
-    'optimizationLevel': optimizationLevel,
+    if (optimizationLevel != null) 'optimizationLevel': optimizationLevel!,
   };
 
 
@@ -56,7 +63,7 @@ class JsCompilerConfig extends WebCompilerConfig {
     this.csp = false,
     this.dumpInfo = false,
     this.nativeNullAssertions = false,
-    super.optimizationLevel = WebCompilerConfig.kDefaultOptimizationLevel,
+    super.optimizationLevel,
     this.noFrequencyBasedMinification = false,
     super.sourceMaps = true,
     super.renderer = WebRendererMode.defaultForJs,
@@ -68,7 +75,6 @@ class JsCompilerConfig extends WebCompilerConfig {
     required WebRendererMode renderer,
   }) : this(
           nativeNullAssertions: nativeNullAssertions,
-          optimizationLevel: WebCompilerConfig.kDefaultOptimizationLevel ,
           renderer: renderer,
         );
 
@@ -108,13 +114,22 @@ class JsCompilerConfig extends WebCompilerConfig {
         if (buildMode == BuildMode.debug) '--enable-asserts',
       ];
 
+  @override
+  int optimizationLevelForBuildMode(BuildMode mode) {
+    final int level = super.optimizationLevelForBuildMode(mode);
+
+    // dart2js optimization level 0 is not well supported. Use
+    // 1 instead.
+    return level == 0 ? 1 : level;
+  }
+
   /// Arguments to use in the full JS compile, but not CFE-only.
   ///
   /// Includes the contents of [toSharedCommandOptions].
   List<String> toCommandOptions(BuildMode buildMode) => <String>[
         if (buildMode != BuildMode.release) '--no-minify',
         ...toSharedCommandOptions(buildMode),
-        '-O$optimizationLevel',
+        '-O${optimizationLevelForBuildMode(buildMode)}',
         if (dumpInfo) '--stage=dump-info-all',
         if (noFrequencyBasedMinification) '--no-frequency-based-minification',
         if (csp) '--csp',
@@ -137,7 +152,7 @@ class JsCompilerConfig extends WebCompilerConfig {
 /// Configuration for the Wasm compiler.
 class WasmCompilerConfig extends WebCompilerConfig {
   const WasmCompilerConfig({
-    super.optimizationLevel = WebCompilerConfig.kDefaultOptimizationLevel,
+    super.optimizationLevel,
     this.stripWasm = true,
     super.sourceMaps = true,
     super.renderer = WebRendererMode.defaultForWasm,
@@ -155,7 +170,7 @@ class WasmCompilerConfig extends WebCompilerConfig {
   List<String> toCommandOptions(BuildMode buildMode) {
     final bool stripSymbols = buildMode == BuildMode.release && stripWasm;
     return <String>[
-      '-O$optimizationLevel',
+      '-O${optimizationLevelForBuildMode(buildMode)}',
       '--${stripSymbols ? '' : 'no-'}strip-wasm',
       if (!sourceMaps) '--no-source-maps',
       if (buildMode == BuildMode.debug) '--extra-compiler-option=--enable-asserts',

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

@@ -83,25 +83,6 @@ void main() {
     ProcessManager: () => processManager,
   });
 
-  testUsingContext('Refuses to build a debug build for web', () async {
-    final CommandRunner<void> runner = createTestCommandRunner(BuildCommand(
-      artifacts: artifacts,
-      androidSdk: FakeAndroidSdk(),
-      buildSystem: TestBuildSystem.all(BuildResult(success: true)),
-      fileSystem: fileSystem,
-      logger: logger,
-      processUtils: processUtils,
-      osUtils: FakeOperatingSystemUtils(),
-    ));
-
-    expect(() => runner.run(<String>['build', 'web', '--debug', '--no-pub']),
-      throwsA(isA<UsageException>()));
-  }, overrides: <Type, Generator>{
-    Platform: () => fakePlatform,
-    FeatureFlags: () => TestFeatureFlags(isWebEnabled: true),
-    ProcessManager: () => processManager,
-  });
-
   testUsingContext('Refuses to build for web when feature is disabled', () async {
     final CommandRunner<void> runner = createTestCommandRunner(BuildCommand(
       artifacts: artifacts,

packages/flutter_tools/test/general.shard/build_system/targets/web_test.dart

@@ -909,7 +909,7 @@ void main() {
         '--no-minify',
         '--no-source-maps',
         '--enable-asserts',
-        '-O4',
+        '-O1',
         '-o',
         environment.buildDir.childFile('main.dart.js').absolute.path,
         environment.buildDir.childFile('app.dill').absolute.path,

packages/flutter_tools/test/general.shard/web/compile_web_test.dart

@@ -107,7 +107,7 @@ void main() {
         label: 'web-compile',
             parameters: CustomDimensions(
               buildEventSettings:
-                  'optimizationLevel: 4; web-renderer: skwasm,canvaskit; web-target: wasm,js;',
+                  'optimizationLevel: 0; web-renderer: skwasm,canvaskit; web-target: wasm,js;',
 
       ),
           ),
@@ -121,7 +121,7 @@ void main() {
         Event.flutterBuildInfo(
           label: 'web-compile',
           buildType: 'web',
-          settings: 'optimizationLevel: 4; web-renderer: skwasm,canvaskit; web-target: wasm,js;',
+          settings: 'optimizationLevel: 0; web-renderer: skwasm,canvaskit; web-target: wasm,js;',
         ),
       ]),
     );