Adding package:code_transformers for unifying common transformers code

pkg/code_transformers/lib/src/resolver_transformer.dart

Index: pkg/code_transformers/lib/src/resolver_transformer.dart
diff --git a/pkg/code_transformers/lib/src/resolver_transformer.dart b/pkg/code_transformers/lib/src/resolver_transformer.dart
new file mode 100644
index 0000000000000000000000000000000000000000..ed8325a86340ac1f5ca0da05a0fc3d5c5d38d104
--- /dev/null
+++ b/pkg/code_transformers/lib/src/resolver_transformer.dart
@@ -0,0 +1,55 @@
+// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library code_transformers.src.resolver_transformer;
+
+import 'dart:async';
+import 'package:barback/barback.dart';
+
+import 'resolver.dart';
+import 'resolver_impl.dart';
+
+/// Filter for whether the specified asset is an entry point to the Dart
+/// application.
+typedef EntryPointFilter(Asset input);
+
+/// Transformer which maintains up-to-date resolved ASTs for the specified
+/// code entry points.
+///
+/// This can used by transformers dependent on resolved ASTs which can reference
+/// this transformer to get the resolver needed.
+///
+/// This transformer must be in a phase before any dependent transformers. The
+/// resolve AST is automatically updated any time any dependent assets are
+/// changed.
+///
+/// This will only resolve the AST for code beginning from assets which are
+/// accepted by [entryPointFilter].
+///
+/// If multiple transformers rely on a resolved AST they should (ideally) share
+/// the same ResolverTransformer to avoid re-parsing the AST.
+class ResolverTransformer extends Transformer {
+  final Map<AssetId, Resolver> _resolvers = {};
+  final EntryPointFilter entryPointFilter;
+  final String dartSdkDirectory;
+
+  ResolverTransformer(this.dartSdkDirectory, this.entryPointFilter);
+
+  Future<bool> isPrimary(Asset input) =>
+      new Future.value(entryPointFilter(input));
+
+  /// Updates the resolved AST for the primary input of the transform.
+  Future apply(Transform transform) {
+    var resolver = getResolver(transform.primaryInput.id);
+
+    return resolver.updateSources(transform).then((_) {
+      transform.addOutput(transform.primaryInput);
+      return null;
+    });
+  }
+
+  /// Get a resolver for the AST starting from [id].
+  Resolver getResolver(AssetId id) =>
+      _resolvers.putIfAbsent(id, () => new ResolverImpl(id, dartSdkDirectory));
+}