[tool][web] Require --release for "flutter run --wasm" and emulate "flutter build web --wasm" #190777

Currently, running flutter run --wasm without --release creates an ambiguous middle ground: it attempts a debug-style compilation without full DDC/DWDS debugging support, yet lacks parity with production Wasm deployments (flutter build web --wasm). There is no clean way for developers to test and preview an actual Wasm production deployment locally without manually performing a build and setting up a separate web server.

We propose:

1. Require --release when using --wasm: flutter run --wasm must require --release. Attempting flutter run --wasm in debug/profile mode will fail fast with a helpful tool exit error.
2. Emulate flutter build web --wasm in flutter run --wasm --release: Provide maximum parity with production builds and local web serving setup (including release compiler optimizations, headers, and asset server setup).
3. Establish Clear Developer Expectations: Hot reload is explicitly unavailable for Wasm release runs, while Hot Restart (R / r) operates on a best-effort basis (triggering full Wasm re-compilation and browser reload).

Proposed Behavior & Scope

1. Command Flag Validation

When executing flutter run --wasm, the CLI tool checks the target build mode:

$ flutter run -d chrome --wasm
Error: --wasm mode for "flutter run" requires --release mode.
To run your web app with WebAssembly, pass --release:
  flutter run -d chrome --wasm --release

2. Parity with flutter build web --wasm Deployment

flutter run --wasm --release will emulate the exact output structure and headers of running flutter build web --wasm + serving via a production web server:

• Release Asset Server: Serves compiled web output via ReleaseAssetServer.
• Cross-Origin Isolation Headers: Automatically injects required Wasm headers:
  • Cross-Origin-Opener-Policy: same-origin
  • Cross-Origin-Embedder-Policy: require-corp (or credentialless)
  • Cross-Origin-Resource-Policy: cross-origin
  • Access-Control-Allow-Origin: *

3. Compiler Parity

Align ResidentWebRunner compiler configuration for Wasm release builds with build_web.dart, producing release-optimized Wasm artifacts (WasmCompilerConfig with release optimization levels, minification, and symbol stripping).

4. Development Loop Expectations

• Hot Reload: Disabled (not supported for static Wasm compilation).
• Hot Restart: Supported on a best-effort basis by re-invoking dart2wasm and refreshing the connected browser window.

Key File Touchpoints in flutter_tools

1. packages/flutter_tools/lib/src/commands/run.dart
   Add validation logic in RunCommand.validateCommand() to reject non-release build modes when useWasm is active.

2. packages/flutter_tools/lib/src/isolated/web_asset_server.dart & packages/flutter_tools/lib/src/isolated/release_asset_server.dart
   Ensure ReleaseAssetServer is used for all flutter run --wasm --release sessions with full cross-origin isolation header support.

3. packages/flutter_tools/lib/src/isolated/resident_web_runner.dart
   Update _compilerConfig in ResidentWebRunner to pass release WasmCompilerConfig options matching build_web.dart and handle best-effort hot restart re-compilation.