[flutter_tools] Add flutter assemble support for tool extension assemble targets - #194563

…and use pathSegments in prototype build

…d and align build APIs

Keep host Source and Target untouched in flutter_tools and convert extension Sources inside ExtensionAssembleTarget, reverting changes to built-in targets and Source.fromProject.

…le integration

- Prefix extension source and artifact types (`ExtensionSource`, `ExtensionArtifact`, `ExtensionHostArtifact`) and make `ExtensionSource` a sealed class.
- Keep `BuildMode` in `flutter_tools_core/lib/src/build.dart` and replace `SimpleTarget` with `BuiltInTargets` string constants.
- Revert host `build_system.dart` changes and rely on `-o` for `flutter assemble` output directories.
- Lazily query extension build targets in `AssembleCommand` only when a requested target is not built in, and remove duplicate Linux target entries.
- Trim AOT prototype targets and fix error handling and output draining in `flutter_tools_extension_linux_prototype`.

…rgetArtifacts

- Ignore `Exception` from `TargetPlatform.fromName(target.targetPlatform)` in `resolveExtensionTargetArtifacts` so custom embedder target platforms work without throwing `ToolExit`.
- Use `continue;` instead of `break;` on `ExtensionPatternSource` in `resolveExtensionTargetArtifacts`.
- Update unit and hermetic tests in `extension_test.dart` and `tool_extensions_build_test.dart`.

…ension PR reviews

- Prefix failure messages in `ExtensionAssembleTarget.build` with the target name and handle empty `errorMessage`.
- Remove `export 'src/build.dart';` from `flutter_tools_extension_linux_prototype.dart` so only `linuxExtensionEntryPoint` is exported.
- Expand Dartdoc on `targetPlatform`, `outputDir`, and `ExtensionSource.pattern`, and include all fields in `ExtensionBuildTarget.toString()`.

…lative extension outputDir

- Define `DependencyResolverCallback` for `ExtensionAssembleTarget._dependencyResolver`.
- Resolve relative `outputDir` paths against `projectDir` in `resolveExtensionOutputDir`.

…xpand source Dartdoc

- Replace top-level `k*Placeholder` constants with `BuiltInPlaceholders`.
- Add `See also:` cross-links on `ExtensionArtifact`, `ExtensionHostArtifact`, `ExtensionSource.pattern`, and `ExtensionPatternSource`.
- Expand Dartdoc on `ExtensionArtifactSource`, `ExtensionHostArtifactSource`, `ExtensionBuildTarget.outputs`, and `ExtensionBuildTarget.outputDir`.

…ild-remaining-slice

# Conflicts:
#	packages/flutter_tools/lib/src/commands/assemble.dart

…arget and update assemble integration

…arget in extension protocol

…7c-assemble-host-integration

# Conflicts:
#	packages/flutter_tools/lib/src/build_system/targets/extension.dart
#	packages/flutter_tools/lib/src/commands/assemble.dart
#	packages/flutter_tools/lib/src/commands/build.dart
#	packages/flutter_tools/packages/flutter_tools_core/lib/src/build.dart
#	packages/flutter_tools/packages/flutter_tools_extension/lib/src/build.dart
#	packages/flutter_tools/test/commands.shard/hermetic/tool_extensions_build_test.dart
#	packages/flutter_tools/test/general.shard/build_system/targets/extension_test.dart

…7c-assemble-host-integration

…7c-assemble-host-integration

# Conflicts:
#	packages/flutter_tools/test/commands.shard/hermetic/tool_extensions_build_test.dart

… ExtensionTarget

Passes the resolved targetPlatform from ExtensionAssembleTarget as the fallback platform when mapping ExtensionArtifactSource to Source.artifact in _toHostSource, and adds test coverage for unknown dependency resolution in AssembleCommand.

…7c-assemble-host-integration

# Conflicts:
#	packages/flutter_tools/lib/src/build_system/targets/extension.dart
#	packages/flutter_tools/lib/src/experimental/extension_build_manager.dart
#	packages/flutter_tools/test/commands.shard/hermetic/tool_extensions_build_test.dart

…UTPUT_DIR}

Makes outputDir required on ExtensionBuildTarget and BuildTarget, requires name, description, and outputDir during JSON deserialization, and updates resolveExtensionOutputDir to resolve {OUTPUT_DIR} from the configured build directory.

Part of flutter#190755

…7c-assemble-host-integration

# Conflicts:
#	packages/flutter_tools/test/commands.shard/hermetic/tool_extensions_build_test.dart

Updates extension build target tests to specify outputDir with {OUTPUT_DIR} and verifies resolveExtensionOutputDir placeholder validation.

Part of flutter#190755

…ild-remaining-slice

# Conflicts:
#	packages/flutter_tools/packages/flutter_tools_core/lib/src/build.dart
#	packages/flutter_tools/packages/flutter_tools_extension/lib/src/build.dart
#	packages/flutter_tools/packages/flutter_tools_extension_linux_prototype/test/linux_build_test.dart

…mble-host-integration

…mble-host-integration

# Conflicts:
#	packages/flutter_tools/lib/src/commands/assemble.dart

Throw a `ToolExit` when an `ExtensionArtifactSource` specifies an explicit `platform` that is not a built-in `TargetPlatform`, or when a target-platform-specific engine artifact resolves to a `null` target platform on an out-of-tree target.