Add support for running dart2wasm in dry run mode on js compilations (#171682)

Example message with findings:

```

Wasm dry run findings:
Found incompatibilities with WebAssembly.

package:counter1/main.dart 3:1 - dart:html unsupported (0)


Consider addressing these issues to enable wasm builds. See docs for more info: https://docs.flutter.dev/platform-integration/web/wasm


Compiling lib/main.dart for the Web...                             10.9s
✓ Built build/web
```

Example message without findings:

```

Wasm dry run succeeded. Consider building and testing your application with the `--wasm` flag. See docs for more info: https://docs.flutter.dev/platform-integration/web/wasm

Compiling lib/main.dart for the Web...                             10.8s
✓ Built build/web
```

---------

Co-authored-by: Nate Biggs <natebiggs@google.com>

Author: biggs0125

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

@@ -351,7 +351,13 @@ class Dart2WasmTarget extends Dart2WebTarget {
       processManager: environment.processManager,
     );
 
-    await processUtils.run(throwOnError: true, compilationArgs);
+    final RunResult runResult = await processUtils.run(
+      throwOnError: !compilerConfig.dryRun,
+      compilationArgs,
+    );
+    if (compilerConfig.dryRun) {
+      _handleDryRunResult(environment, runResult);
+    }
   }
 
   @override
@@ -361,31 +367,68 @@ class Dart2WasmTarget extends Dart2WebTarget {
   List<String> get depfiles => const <String>['dart2wasm.d'];
 
   @override
-  Map<String, Object?> get buildConfig => <String, Object?>{
-    'compileTarget': 'dart2wasm',
-    'renderer': compilerConfig.renderer.name,
-    'mainWasmPath': 'main.dart.wasm',
-    'jsSupportRuntimePath': 'main.dart.mjs',
-  };
-
-  @override
-  Iterable<File> buildFiles(Environment environment) => environment.buildDir
-      .listSync(recursive: true)
-      .whereType<File>()
-      .where(
-        (File file) => switch (file.basename) {
-          'main.dart.wasm' || 'main.dart.mjs' => true,
-          'main.dart.wasm.map' => compilerConfig.sourceMaps,
-          _ => false,
-        },
+  Map<String, Object?> get buildConfig => compilerConfig.dryRun
+      ? const <String, Object?>{}
+      : <String, Object?>{
+          'compileTarget': 'dart2wasm',
+          'renderer': compilerConfig.renderer.name,
+          'mainWasmPath': 'main.dart.wasm',
+          'jsSupportRuntimePath': 'main.dart.mjs',
+        };
+
+  @override
+  Iterable<File> buildFiles(Environment environment) => compilerConfig.dryRun
+      ? const <File>[]
+      : environment.buildDir
+            .listSync(recursive: true)
+            .whereType<File>()
+            .where(
+              (File file) => switch (file.basename) {
+                'main.dart.wasm' || 'main.dart.mjs' => true,
+                'main.dart.wasm.map' => compilerConfig.sourceMaps,
+                _ => false,
+              },
+            );
+
+  @override
+  Iterable<String> get buildPatternStems => compilerConfig.dryRun
+      ? const <String>[]
+      : <String>[
+          'main.dart.wasm',
+          'main.dart.mjs',
+          if (compilerConfig.sourceMaps) 'main.dart.wasm.map',
+        ];
+
+  void _handleDryRunResult(Environment environment, RunResult runResult) {
+    final int exitCode = runResult.exitCode;
+    final String stdout = runResult.stdout;
+    final String stderr = runResult.stderr;
+    if (exitCode != 0 && exitCode != 254) {
+      environment.logger.printWarning('Unexpected wasm dry run failure ($exitCode):');
+      if (stderr.isNotEmpty) {
+        environment.logger.printWarning(stdout);
+        environment.logger.printWarning(stderr);
+      }
+    } else if (exitCode == 0) {
+      environment.logger.printWarning(
+        'Wasm dry run succeeded. Consider building and testing your application with the '
+        '`--wasm` flag. See docs for more info: '
+        'https://docs.flutter.dev/platform-integration/web/wasm',
       );
-
-  @override
-  Iterable<String> get buildPatternStems => <String>[
-    'main.dart.wasm',
-    'main.dart.mjs',
-    if (compilerConfig.sourceMaps) 'main.dart.wasm.map',
-  ];
+    } else if (stderr.isNotEmpty) {
+      environment.logger.printWarning('Wasm dry run failed:');
+      environment.logger.printWarning(stdout);
+      environment.logger.printWarning(stderr);
+    } else if (stdout.isNotEmpty) {
+      environment.logger.printWarning('Wasm dry run findings:');
+      environment.logger.printWarning(stdout);
+      environment.logger.printWarning(
+        'Consider addressing these issues to enable wasm builds. See docs for more info: '
+        'https://docs.flutter.dev/platform-integration/web/wasm\n',
+      );
+    }
+    environment.logger.printWarning('Use --no-wasm-dry-run to disable these warnings.');
+  }
 }
 
 /// Unpacks the dart2js or dart2wasm compilation and resources to a given
@@ -760,7 +803,7 @@ class WebServiceWorker extends Target {
     final String serviceWorker = generateServiceWorker(fileGeneratorsPath, urlToHash, <String>[
       'main.dart.js',
       if (compileConfigs.any(
-        (WebCompilerConfig config) => config is WasmCompilerConfig,
+        (WebCompilerConfig config) => config is WasmCompilerConfig && !config.dryRun,
       )) ...<String>['main.dart.wasm', 'main.dart.mjs'],
       'index.html',
       'flutter_bootstrap.js',

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

@@ -112,6 +112,13 @@ class BuildWebCommand extends BuildSubCommand {
           'If not explicitly set, uses the compilation mode (debug, profile, release).',
       hide: !verboseHelp,
     );
+    argParser.addFlag(
+      'wasm-dry-run',
+      defaultsTo: true,
+      help:
+          'Compiles wasm in dry run mode during JS only compilations. '
+          'Disable to suppress warnings.',
+    );
     argParser.addFlag(
       'no-frequency-based-minification',
       negatable: false,
@@ -226,6 +233,13 @@ class BuildWebCommand extends BuildSubCommand {
           sourceMaps: sourceMaps,
           renderer: webRenderer,
         ),
+        WasmCompilerConfig(
+          optimizationLevel: optimizationLevel,
+          stripWasm: boolArg('strip-wasm'),
+          sourceMaps: sourceMaps,
+          minify: minifyWasm,
+          dryRun: boolArg('wasm-dry-run'),
+        ),
       ];
     }
 

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

@@ -147,6 +147,7 @@ class WasmCompilerConfig extends WebCompilerConfig {
     super.optimizationLevel,
     this.stripWasm = true,
     this.minify,
+    this.dryRun = false,
     super.sourceMaps = true,
     super.renderer = WebRendererMode.defaultForWasm,
   });
@@ -159,6 +160,8 @@ class WasmCompilerConfig extends WebCompilerConfig {
 
   final bool? minify;
 
+  final bool dryRun;
+
   @override
   CompileTarget get compileTarget => CompileTarget.wasm;
 
@@ -184,6 +187,7 @@ class WasmCompilerConfig extends WebCompilerConfig {
       if (!sourceMaps) '--no-source-maps',
       if (minify ?? buildMode == BuildMode.release) '--minify' else '--no-minify',
       if (buildMode == BuildMode.debug) '--extra-compiler-option=--enable-asserts',
+      if (dryRun) '--extra-compiler-option=--dry-run',
     ];
   }
 
@@ -193,8 +197,15 @@ class WasmCompilerConfig extends WebCompilerConfig {
       ...super._buildKeyMap,
       kStripWasm: stripWasm,
       'minify': minify,
+      'dryRun': dryRun,
       WebCompilerConfig.kSourceMapsEnabled: sourceMaps,
     };
     return jsonEncode(settings);
   }
+
+  @override
+  Map<String, Object> get buildEventAnalyticsValues => <String, Object>{
+    ...super.buildEventAnalyticsValues,
+    'dryRun': dryRun,
+  };
 }

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

@@ -412,17 +412,29 @@ void main() {
           TestBuildSystem.all(BuildResult(success: true), (Target target, Environment environment) {
             expect(target, isA<WebServiceWorker>());
             final List<WebCompilerConfig> configs = (target as WebServiceWorker).compileConfigs;
-            expect(configs, hasLength(1));
-            final WebCompilerConfig config = configs.single;
-            expect(config.renderer, WebRendererMode.canvaskit);
-            expect(config.compileTarget, CompileTarget.js);
-            final List<String> options = config.toCommandOptions(BuildMode.release);
-            expect(options, <String>[
+            expect(configs, hasLength(2));
+            final WebCompilerConfig jsConfig = configs[0];
+            expect(jsConfig.renderer, WebRendererMode.canvaskit);
+            expect(jsConfig.compileTarget, CompileTarget.js);
+            final List<String> jsOptions = jsConfig.toCommandOptions(BuildMode.release);
+            expect(jsOptions, <String>[
               '--native-null-assertions',
               '--no-source-maps',
               '-O4',
               '--minify',
             ]);
+
+            final WebCompilerConfig wasmConfig = configs[1];
+            expect(wasmConfig.renderer, WebRendererMode.skwasm);
+            expect(wasmConfig.compileTarget, CompileTarget.wasm);
+            final List<String> wasmOptions = wasmConfig.toCommandOptions(BuildMode.release);
+            expect(wasmOptions, <String>[
+              '-O2',
+              '--strip-wasm',
+              '--no-source-maps',
+              '--minify',
+              '--extra-compiler-option=--dry-run',
+            ]);
           }),
     },
   );

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

@@ -1255,12 +1255,14 @@ name: foo
       WasmCompilerConfig(renderer: WebRendererMode.canvaskit),
       WasmCompilerConfig(stripWasm: false),
       WasmCompilerConfig(minify: false),
+      WasmCompilerConfig(dryRun: true),
 
       // All properties non-default
       WasmCompilerConfig(
         optimizationLevel: 0,
         stripWasm: false,
         renderer: WebRendererMode.canvaskit,
+        dryRun: true,
       ),
     ];
 

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

@@ -105,7 +105,8 @@ environement:
           Event.flutterBuildInfo(
             label: 'web-compile',
             buildType: 'web',
-            settings: 'optimizationLevel: 0; web-renderer: skwasm,canvaskit; web-target: wasm,js;',
+            settings:
+                'dryRun: false; optimizationLevel: 0; web-renderer: skwasm,canvaskit; web-target: wasm,js;',
           ),
         ]),
       );