Update browser_environment.js to require try_table - #193090

…trap.js (flutter#193271)

Synchronizes Flutter Web's Wasm feature detection with the Dart SDK's
`dart2wasm` compiler output (`main.dart.support.js`) and automates
keeping `supportsDart2Wasm()` in sync during Dart SDK rolls.

Follows up on flutter#193090.

### Background

When `dart2wasm` compiles a Wasm module
(`pkg/dart2wasm/lib/compile.dart`), it emits `<output>.support.js`
(`main.dart.support.js` in Flutter Web builds), containing a JavaScript
boolean expression that checks whether the current browser supports all
WebAssembly features required by the compiled module (WasmGC,
relaxed/standard SIMD, `js-string` builtins, `try_table`, and
conditional `requiresMultiMemory`).

Previously,
`engine/src/flutter/lib/web_ui/flutter_js/src/browser_environment.js`
maintained a manual copy of this byte array check inside
`supportsDart2Wasm()`. Whenever `dart2wasm` added or modified a required
Wasm feature probe in the Dart SDK, `flutter_js` had to be manually
updated in a separate PR. Additionally, `main.dart.support.js` emitted
by `dart2wasm` during `flutter build web --wasm` was never read by
`flutter_tools`.

### Changes

1. **Consume `main.dart.support.js` in `flutter_tools` bootstrap
generation**:
- `generateFlutterBootstrapScript` reads `main.dart.support.js` when
present in the build output directory and emits
`_flutter.supportsDart2Wasm = <expr>;` immediately after
`_flutter.buildConfig = ...;` (preserving strict JSON parsing in
`--web-content-hash`).
- `supportsDart2Wasm()` in `browser_environment.js` evaluates
`window._flutter?.supportsDart2Wasm ?? defaultSupportsDart2Wasm()` after
enforcing the Firefox `< 147` guard, so standard `flutter build web
--wasm` apps use the exact feature probe emitted by the active
`dart2wasm` compiler (including conditional flags such as
`--extra-compiler-option=--import-shared-memory`), while custom HTML
embeddings using standalone `flutter.js` fall back to
`defaultSupportsDart2Wasm()` from `supports_dart2wasm.js`.
2. **Automate `supports_dart2wasm.js` generation during Dart SDK
rolls**:
- Moves the default `supportsDart2Wasm` probe into a standalone
generated module
`engine/src/flutter/lib/web_ui/flutter_js/src/supports_dart2wasm.js`
imported by `browser_environment.js`.
- `engine/src/tools/dart/create_updated_flutter_deps.py`
(`SyncSupportsDart2WasmJs`) parses `_generateSupportJs` from
`pkg/dart2wasm/lib/compile.dart` (from local `third_party/dart` or
`dart.googlesource.com` when `--dart_revision` is specified) and
generates `supports_dart2wasm.js`.
- `.github/workflows/roll-dart-dependencies.yml` stages
`engine/src/flutter/lib/web_ui/flutter_js/src/supports_dart2wasm.js`
alongside `DEPS` when rolling Dart dependencies.
3. **CI Parity Guard**:
- `engine/src/tools/dart/create_updated_flutter_deps_tests.py` verifies
`ExtractDart2WasmSupportExpression` across historical `dart2wasm` epochs
and asserts that `supports_dart2wasm.js` matches
`third_party/dart/pkg/dart2wasm/lib/compile.dart` whenever
`third_party/dart` is present.

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] All existing and new tests are passing.