Refactoring resolvers to make them able to be run in parallel.

pkg/code_transformers/lib/src/resolver.dart

 // 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_transformer.src.resolver;
 
 import 'package:analyzer/src/generated/element.dart';
 import 'package:barback/barback.dart';
 import 'package:source_maps/refactor.dart';
 import 'package:source_maps/span.dart' show SourceFile, Span;
 
 
 /// Class for working with a barback based resolved AST.
 abstract class Resolver {
   /// The Dart entry point file where parsing begins.
   AssetId get entryPoint;
 
+  /// Update the status of all the sources referenced by the entryPoint and
+  /// update the resolved library.
+  ///
+  /// [release] must be called when done handling this Resolver to allow it
+  /// to be used by later phases.
+  Future<Resolver> resolve(Transform transform);
+
+  /// Release this resolver so it can be updated by following transforms.
+  void release();
+
   /// Gets the resolved Dart library for the entry asset, or null if
   /// the AST has not been resolved.
   ///
   /// If the AST has not been resolved then this normally means that the
   /// transformer hosting this needs to be in an earlier phase.
   LibraryElement get entryLibrary;
 
   /// Gets all libraries accessible from the entry point, recursively.
   ///
   /// This includes all Dart SDK libraries as well.
@@ skipping 44 matching lines @@
   Span getSourceSpan(Element element);
 
   /// Creates a text edit transaction for the given element if it is able
   /// to be edited, returns null otherwise.
   ///
   /// The transaction contains the entire text of the source file where the
   /// element originated. If the element was from a library part then the
   /// source file is the part file rather than the library.
   TextEditTransaction createTextEditTransaction(Element element);
 }