feature: add build.yaml setup for CombiningBuilder (dart-lang/source_…

…gen#350)

work towards dart-lang/source_gen#319

pkgs/source_gen/source_gen/build.yaml

@@ -0,0 +1,9 @@
+# Read about `build.yaml` at https://pub.dartlang.org/packages/build_config
+builders:
+  combining_builder:
+    import: "package:source_gen/builder.dart"
+    builder_factories: ["combiningBuilder"]
+    build_extensions: {".dart": [".g.dart"]}
+    auto_apply: none
+    build_to: source
+    required_inputs: ['.g.part']

pkgs/source_gen/source_gen/lib/builder.dart

@@ -24,6 +24,17 @@ import 'src/builder.dart';
 const _outputExtensions = '.g.dart';
 const _partFiles = '.g.part';
 
+Builder combiningBuilder([BuilderOptions options]) {
+  if (options != null && options.config.isNotEmpty) {
+    if (log == null) {
+      throw new StateError('Upgrade `build_runner` to at least 0.8.2.');
+    } else {
+      log.warning('These options were ignored: `${options.config}`.');
+    }
+  }
+  return const CombiningBuilder();
+}
+
 /// A [Builder] which combines part files generated from [SharedPartBuilder].
 ///
 /// This will glob all files of the form `.*.g.part`.