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

pkg/code_transformers/lib/src/resolver.dart

Index: pkg/code_transformers/lib/src/resolver.dart
diff --git a/pkg/code_transformers/lib/src/resolver.dart b/pkg/code_transformers/lib/src/resolver.dart
index 80e385fd4e39db879a03e49a6b13f858735c41af..f520efce6a77a785d23f35fa90ee1fe4d388aeac 100644
--- a/pkg/code_transformers/lib/src/resolver.dart
+++ b/pkg/code_transformers/lib/src/resolver.dart
@@ -15,6 +15,16 @@ 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.
   ///