Change the default optimization level to -O2 for wasm in release mode. (flutter#162917)

As per the investigation in
flutter#162620, we determined that the
soundness we gain from using `-O2` outweighs the perf benefits of `-O4`.

Author: eyebrowsoffire

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

@@ -21,15 +21,9 @@ sealed class WebCompilerConfig {
   /// Build environment flag for [sourceMaps].
   static const String kSourceMapsEnabled = 'SourceMaps';
 
-  /// Calculates the optimization level for dart2js/dart2wasm for the given
+  /// Calculates the optimization level for the compiler 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'),
-      };
+  int optimizationLevelForBuildMode(BuildMode mode);
 
   /// The compiler optimization level specified by the user.
   ///
@@ -111,13 +105,15 @@ class JsCompilerConfig extends WebCompilerConfig {
   ];
 
   @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;
-  }
+  int optimizationLevelForBuildMode(BuildMode mode) =>
+      optimizationLevel ??
+      switch (mode) {
+        // dart2js optimization level 0 is not well supported. Use
+        // 1 instead.
+        BuildMode.debug => 1,
+        BuildMode.profile || BuildMode.release => 4,
+        BuildMode.jitRelease => throw ArgumentError('Invalid build mode for web'),
+      };
 
   /// Arguments to use in the full JS compile, but not CFE-only.
   ///
@@ -163,6 +159,19 @@ class WasmCompilerConfig extends WebCompilerConfig {
   @override
   CompileTarget get compileTarget => CompileTarget.wasm;
 
+  @override
+  int optimizationLevelForBuildMode(BuildMode mode) =>
+      optimizationLevel ??
+      switch (mode) {
+        BuildMode.debug => 0,
+
+        // The optimization level of O2 uses only sound optimizations. We default
+        // to this level because our web benchmarks have shown that the difference
+        // between O2 and O4 is marginal enough that we would prefer soundness here.
+        BuildMode.profile || BuildMode.release => 2,
+        BuildMode.jitRelease => throw ArgumentError('Invalid build mode for web'),
+      };
+
   List<String> toCommandOptions(BuildMode buildMode) {
     final bool stripSymbols = buildMode == BuildMode.release && stripWasm;
     return <String>[

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

@@ -1140,7 +1140,7 @@ void main() {
     WebRendererMode.canvaskit,
     WebRendererMode.skwasm,
   ]) {
-    for (int level = 1; level <= 4; level++) {
+    for (final int? level in <int?>[null, 0, 1, 2, 3, 4]) {
       for (final bool strip in <bool>[true, false]) {
         for (final List<String> defines in const <List<String>>[
           <String>[],
@@ -1151,6 +1151,13 @@ void main() {
               test(
                 'Dart2WasmTarget invokes dart2wasm with renderer=$renderer, -O$level, stripping=$strip, defines=$defines, modeMode=$buildMode sourceMaps=$sourceMaps',
                 () => testbed.run(() async {
+                  final int expectedLevel =
+                      level ??
+                      switch (buildMode) {
+                        'debug' => 0,
+                        'profile' || 'release' => 2,
+                        _ => throw UnimplementedError(),
+                      };
                   environment.defines[kBuildMode] = buildMode;
                   environment.defines[kDartDefines] = encodeDartDefines(defines);
 
@@ -1182,7 +1189,7 @@ void main() {
                         ],
                         '-DFLUTTER_WEB_CANVASKIT_URL=https://www.gstatic.com/flutter-canvaskit/abcdefghijklmnopqrstuvwxyz/',
                         '--extra-compiler-option=--depfile=${depFile.absolute.path}',
-                        '-O$level',
+                        '-O$expectedLevel',
                         if (strip && buildMode == 'release') '--strip-wasm' else '--no-strip-wasm',
                         if (!sourceMaps) '--no-source-maps',
                         if (buildMode == 'debug') '--extra-compiler-option=--enable-asserts',