[native_assets_builder] Introduce ProtocolExtension

[dcharkes]

For every extension to the protocol, the SDK authors have to pass 5 pieces of information currently:

    final BuildResult? buildResult = await buildRunner.build(
      buildAssetTypes: <String>[CodeAsset.type],
      inputCreator:
          () =>
              BuildInputBuilder()
                ..config.setupCode(
                  targetArchitecture: architecture,
                  linkModePreference: LinkModePreference.dynamic,
                  cCompiler: cCompilerConfig,
                  targetOS: targetOS!,
                  android: androidConfig,
                  iOS: iosConfig,
                  macOS: macOSConfig,
                ),
      inputValidator:
          (BuildInput config) async => <String>[...await validateCodeAssetBuildInput(config)],
      buildValidator:
          (BuildInput config, BuildOutput output) async => <String>[
            ...await validateCodeAssetBuildOutput(config, output),
          ],
      applicationAssetValidator:
          (List<EncodedAsset> assets) async => <String>[
            ...await validateCodeAssetInApplication(assets),
          ],

https://github.com/flutter/flutter/blob/d452d04a07f3c37d3ed60989e01e64e0f5c34913/packages/flutter_tools/lib/src/isolated/native_assets/native_assets.dart#L604-L626

Once we also add syntax validators next to semantic validators, it will be 7 pieces of information for the build invocation and 7 slightly different pieces of information for the link invocation.

This doesn't scale.

We should introduce a concept of a protocol extension that will ensure either all or none setup/validate callbacks are passed in to the native assets builder.

A sketch:

/// An extension to the base protocol for `hook/build.dart` and
/// `hook/link.dart`.
///
/// The extension contains callbacks to
/// 1. setup the config,
/// 2. validate the protocol syntax, and
/// 3. validate semantic constraints.
abstract interface class ProtocolExtension {
  /// The [HookConfig.buildAssetTypes] this extension adds.
  List<String> get buildAssetTypes;

  /// Setup the [BuildConfig] for this extension.
  void setupBuildInput(BuildInputBuilder input);

  /// Setup the [HookConfig] for this extension.
  void setupLinkInput(LinkInputBuilder input);

  /// Reports syntactic errors from this extension on the [BuildInput].
  ValidationErrors validateBuildInputSyntax(Map<String, Object?> json);

  /// Reports syntactic errors from this extension on the [LinkInput].
  ValidationErrors validateLinkInputSyntax(Map<String, Object?> json);

  /// Reports syntactic errors from this extension on the [BuildOutput].
  ValidationErrors validateBuildOutputSyntax(Map<String, Object?> json);

  /// Reports syntactic errors from this extension on the [LinkOutput].
  ValidationErrors validateLinkOutputSyntax(Map<String, Object?> json);

  /// Reports semantic errors from this extension on the [BuildInput].
  ///
  /// Only run if [validateBuildInputSyntax] reports no errors.
  Future<ValidationErrors> validateBuildInput(BuildInput input);

  /// Reports semantic errors from this extension on the [LinkInput].
  ///
  /// Only run if [validateLinkInputSyntax] reports no errors.
  Future<ValidationErrors> validateBuildOutput(
    BuildInput input,
    BuildOutput output,
  );

  /// Reports semantic errors from this extension on the [LinkInput].
  ///
  /// Only run if [validateLinkInputSyntax] reports no errors.
  Future<ValidationErrors> validateLinkInput(LinkInput input);

  /// Reports semantic errors from this extension on the [LinkOutput].
  ///
  /// Only run if [validateLinkOutputSyntax] reports no errors.
  Future<ValidationErrors> validateLinkOutput(
    LinkInput input,
    LinkOutput output,
  );

  /// Reports errors on the complete set of assets after all hooks are run.
  ///
  /// Can be used to validate that there are no asset-id or shared library name
  /// collisions.
  ///
  /// Only run if [validateBuildOutputSyntax] and [validateLinkOutputSyntax]
  /// report no errors.
  Future<ValidationErrors> validateApplicationAssets(List<EncodedAsset> assets);
}

@mosuem Would this fit well with the FontAssets you've been looking at? What is the JSON structure and semantic API you've come up with so far?