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

[Markzipan]

Changes at a glance:

DWDS

• ddc_uri_translator.dart
  • Added DdcUriTranslator, which translates paths between FES and package:build asset URI formats (e.g., mapping .dart.lib suffixes to .ddc, handling package lib/ segment inclusion/exclusion, etc.).
• frontend_server_strategy_provider.dart
  • Added FrontendServerBuildDaemonStrategyProvider to translate/remap FES and build daemon paths and other conventions.
• daemon_expression_compiler.dart
  • Added DaemonExpressionCompiler, which forwards expression evaluation requests to the instantiated Frontend Server worker via socket.
• location.dart
  • Updated sourcemap parsing to account for FES-specific URIs (org-dartlang-app and other bits).

Webdev and Serving Workflow

• 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 allowing communication with the build daemon.
• webdev_server.dart
  • Use FrontendServerBuildDaemonStrategyProvider when webHotReload. Also write a special fes_manager_config to signal to build runner that a shared FES should be used.

Tests

• context.dart
  • Rewrote _createBuildRunnerDdcLibraryBundleAssetHandler to serve compiled outputs directly from the build cache or scratch space via multi-roots.
• Migrated a bunch of integration tests to support/run under the new DDC Library Bundle + Frontend Server.
• Updated hot reload tests to signal reload failure by a crash when reading reloaded_source.json. This is unlike a real app but works well for the test fixtures.
• Bumped build_web_compilers to 4.8.2 in test fixtures.

Depends on dart-lang/build#4979