Fix bug in code_transformers. Apparently mock SDK needs to have a source factory

pkg/code_transformers/lib/src/resolvers.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_transformers.src.resolvers;
 
 import 'dart:async';
 import 'package:barback/barback.dart';
 
 import 'package:analyzer/src/generated/sdk.dart' show DartSdk;
@@ skipping 21 matching lines @@
 
   factory Resolvers(dartSdkDirectory) {
     var sdk = new DirectoryBasedDartSdkProxy(dartSdkDirectory);
     var uriResolver = new DartUriResolverProxy(sdk);
     return new Resolvers.fromSdk(sdk, uriResolver);
   }
 
   factory Resolvers.fromMock(Map<String, String> sources,
       {bool reportMissing: false}) {
     var sdk = new MockDartSdk(sources, reportMissing: reportMissing);
-    return new Resolvers.fromSdk(sdk, new DartUriResolver(sdk));
+    return new Resolvers.fromSdk(sdk, sdk.resolver);
   }
 
   /// Get a resolver for [transform]. If provided, this resolves the code
   /// starting from each of the assets in [entryPoints]. If not, this resolves
   /// the code starting from `transform.primaryInput.id` by default.
   ///
   /// [Resolver.release] must be called once it's done being used, or
   /// [ResolverTransformer] should be used to automatically release the
   /// resolver.
   Future<Resolver> get(Transform transform, [List<AssetId> entryPoints]) {
@@ skipping 60 matching lines @@
           resolver.release();
         });
     });
   }
 
   /// Invoked when the resolver is ready to be processed.
   ///
   /// Return a Future to indicate when apply is completed.
   applyResolver(Transform transform, Resolver resolver);
 }