feat(web): resolve raw asset keys via content-hashed manifests and unhide --web-content-hash

[kevmoo]

Description

Completes the --web-content-hash architecture by adding runtime raw-key asset resolution in web_ui, achieving 100% content-hashing coverage across all files in build/web/assets/ (eliminating manifest and shader exclusions), preserving AssetManifest.listAssets() and AssetMetadata.main, validating custom web/flutter_bootstrap.js / web/index.html templates, and un-hiding --web-content-hash (--no-web-content-hash) in flutter build web --help.

Why this is needed

Phase 2 (#191919) and Phase 3 (#192953) renamed physical asset files on disk (<name>.<hash>.<ext>), rewrote target values inside AssetManifest.bin, AssetManifest.bin.json, AssetManifest.json, and FontManifest.json, and emitted precache_manifest.json. Two critical gaps remained before exposing --web-content-hash to users:

1. Cold-cache 404s on raw-key asset requests:
   • While Image.asset resolves variant paths via AssetManifest before calling rootBundle.load(variant.key), direct callers of rootBundle.load(key) / rootBundle.loadString(key) (PlatformAssetBundle) and third-party web plugins (vector_graphics / flutter_svg AssetBytesLoader, lottie, rive, video_player_web, audioplayers_web) pass the raw declared key (assets/data/config.json) directly to ui_web.AssetManager.getAssetUrl(String asset).
   • Without runtime key translation in ui_web.AssetManager.getAssetUrl(asset), raw-key asset fetches requested un-hashed paths (assets/assets/data/config.json) and 404'd on cold caches.
2. Un-hashed manifest and shader files under build/web/assets/:
   • AssetManifest.*, FontManifest.json, NOTICES, NOTICES.Z, and shaders/** were previously excluded from hashing, which meant a single "source": "assets/**" -> Cache-Control: public, max-age=31536000, immutable CDN rule would freeze AssetManifest.bin.json and FontManifest.json across deployments (v1 -> v2).

What this PR changes

1. 100% build/web/assets/ Immutability & Clean AssetManifest.listAssets() (packages/flutter_tools/lib/src/web/content_hash.dart):
   • Removes _excludedFilenames and _excludedDirPrefixes so every file under build/web/assets/ (including shaders/**, NOTICES, NOTICES.Z, FontManifest.json, AssetManifest.json, AssetManifest.bin, and AssetManifest.bin.json) is content-hashed on disk.
   • Keeps AssetManifest.bin and AssetManifest.json strictly limited to user/package manifest entries (storing Uri.decodeFull variant paths so Image.asset on spaced filenames encodes cleanly to %2520 rather than triple-encoding to %252520), and passes non-manifest SDK files (NOTICES, shaders/ink_sparkle.frag, shaders/stretch_effect.frag, AssetManifest.bin, AssetManifest.json) via _flutter.buildConfig.extraAssets (~200 bytes) alongside "assetManifest" and "fontManifest".
   • Sorts discovered asset paths deterministically before manifest assembly so incremental and clean builds emit identical manifest hashes.
   • Fails fast (throwToolExit) if custom web/flutter_bootstrap.js or web/index.html templates omit {{flutter_build_config}} or contain malformed _flutter.buildConfig assignments.
2. Preserve AssetMetadata.main (packages/flutter/lib/src/services/asset_manifest.dart):
   • Updates _AssetManifestBin (asset_manifest.dart) so AssetMetadata.main remains true for the primary 1.0x asset variant when content hashes are embedded in variant filenames.
3. Incremental Build .filecache Tracking (packages/flutter_tools/lib/src/build_system/targets/web.dart):
   • Declares {OUTPUT_DIR}/*/index.html and {OUTPUT_DIR}/flutter_bootstrap.js in WebReleaseBundle.inputs and WebReleaseBundle.outputs when webContentHash is enabled so .filecache records post-injection file hashes.
4. Synchronous Engine-Private Runtime Key Translation (web_ui):
   • Extends FlutterJS with JSBuildConfig (assetManifest, fontManifest, extraAssets) in js_loader.dart.
   • Updates fetchFontManifest in fonts.dart to fetch flutter?.buildConfig?.fontManifest?.toDart ?? 'FontManifest.json'.
   • Adds _loadContentHashedAssetManifest(ui_web.assetManager) inside initializeEngineServices() Future.wait in initialization.dart to register assetManifest, fontManifest, extraAssets, and decoded AssetManifest.bin.<hash>.json mappings in engine-private setContentHashedAssetMap / resolveContentHashedAsset (keeping the public dart:ui_web AssetManager API unchanged).
   • Resolves logical keys and variant paths synchronously inside ui_web.AssetManager.getAssetUrl(String asset).
5. Un-hide --web-content-hash (packages/flutter_tools/lib/src/web/web_options.dart & build_web.dart):
   • Removes hide: true from WebOptions.webContentHash and notes that local canvaskit/** (--no-web-resources-cdn) remains un-hashed (urlHashed: false in precache_manifest.json).

Related Issues

• Fixes [web] Support content-hashed static assets in AssetManifest (--web-content-hash) #191915
• Fixes [web] Support runtime content-hashed asset resolution in web_ui and un-hide --web-content-hash #193031
• Part of [WEB] ☂️ Web Cache invalidation based on pubspec.yaml version #149031

Tests

• packages/flutter_tools/test/general.shard/build_system/targets/web_test.dart
• packages/flutter_tools/test/commands.shard/hermetic/build_web_test.dart & args_test.dart
• packages/flutter/test/services/asset_manifest_test.dart
• engine/src/flutter/lib/web_ui/test/engine/assets_test.dart

Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I signed the [CLA].
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is [test-exempt].
• All existing and new tests are passing.

[gemini-code-assist]

Code Review

This pull request implements runtime asset resolution for web content hashing in Flutter. It updates the web engine to load content-hashed manifests and extra assets from _flutter.buildConfig, and modifies flutter_tools to inject these hashed filenames during release builds. Feedback on the changes suggests supporting single quotes and backticks when parsing JavaScript string literals in _findMatchingClosingBrace, updating the glob patterns in WebReleaseBundle to match the root index.html file, and optimizing JSON decoding in the engine by fusing the UTF-8 and JSON decoders to avoid intermediate string allocations.

[kevmoo]

@mdebbar one more nod, please!