Adding support for Build Daemon + Frontend Server with expression eval.

[Markzipan]

Changes at a glance

DWDS

• asset_scheme.dart: Added AssetScheme to handle platform-specific file extensions (e.g., .ddc.js vs .dart.lib.js, .ddc.dill vs .dart.lib.dill) with BuildRunnerAssetScheme and FrontendServerAssetScheme default impls.
• asset_reader.dart: Removed PackageUriMapper in favor of PathResolver interface with default impls: FrontendServerPathResolver, BuildRunnerPathResolver, and FlutterPathResolver.
• Created web_path_translator.dart to hold asset translation rules (lib/ segments, asset-scheme-specific renaming, etc.).
• loader.dart: Added MetadataLoader (with default impl MergedMetadataLoader) to allow platform-specific reading/parsing of metadata files.
• strategy.dart: LoadStrategy now requires an AssetScheme and accepts a MetadataLoader. BuildSettings now accepts useDebuggerModuleNames.
• dwds.dart: Now exports: AssetScheme, MetadataLoader, and related impls.
• frontend_server_strategy_provider.dart: Added FrontendServerBuildDaemonStrategyProvider to support Frontend Server + Build Daemon config with hot reload.
• daemon_expression_compiler.dart: Added DaemonExpressionCompiler to forward expression evaluation requests directly to the instantiated Frontend Server worker via socket.
• location.dart: Updated sourcemap parsing to account for FES-specific URIs.

Webdev and Serving

• dev_workflow.dart: Spawns a background build_frontend_server:fes_manager subprocess when webHotReload is enabled. This creates a persistent shared Frontend Server (with a persistent scratch space) that can receive expression eval, hot reload, and hot restart requests while communicating with the build daemon.
• webdev_server.dart: Uses FrontendServerBuildDaemonStrategyProvider when webHotReload is enabled. Writes fes_manager_config to signal to build runner that a shared FES should be used. Now passes PathResolver and BuildSettings.
• app_domain.dart: Now supports hot reload requests (instead of rejecting them).
• server_manager.dart: Passes BuildDaemonClient to WebDevServer for build results to be accessible.

Tests

• context.dart: Test fixtures now specify a PathResolver. Rewrote _createBuildRunnerDdcLibraryBundleAssetHandler to serve compiled outputs directly from the build cache or scratch space.
• Now serves .ddc_merged_metadata via the fes_manager.
• Now sends a BuildStatus.succeeded message early to avoid blocking downstream completers and locking.
• Migrated several integration tests to support/run under the DDC Library Bundle + Frontend Server config (split between AMD and DDC Library Bundle).
• Updated hot reload tests to signal reload failure by a crash when reading reloaded_sources.json. This is unlike a real app but works well for the test fixtures.
• Bumped build_web_compilers to 4.8.7 in test fixtures.

Test Validation

• Internal: cl/951722126
• Flutter: TEST: Testing DWDS integration at webdev PR 2836 flutter/flutter#189889

[nshahan]

Since this PR is touching a lot of stuff, I added @srujzs to get another set of eyes on it just in case.

[nshahan]

I know you are doing some manual testing, but are those at a point that you can link some CLs/PRs so we can track? I want to avoid landing this too early if we still have work to do before rolling into the Dart SDK, Flutter, and google3.

[srujzs]

Most of my comments are centered around some of the classes we're adding here.

[Markzipan]

@nshahan I've added my Flutter PR + internal CL to the description; they should be up to date with this change.