Don't add sources to DartSdk contexts.

pkg/analyzer/test/src/mock_sdk.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 analyzer.test.src.mock_sdk;
 
 import 'package:analyzer/file_system/file_system.dart' as resource;
 import 'package:analyzer/file_system/memory_file_system.dart' as resource;
 import 'package:analyzer/src/context/context.dart';
 import 'package:analyzer/src/generated/engine.dart';
@@ skipping 191 matching lines @@
       provider.newFile(library.path, library.content);
     });
   }
 
   @override
   AnalysisContext get context {
     if (_analysisContext == null) {
       _analysisContext = new SdkAnalysisContext();
       SourceFactory factory = new SourceFactory([new DartUriResolver(this)]);
       _analysisContext.sourceFactory = factory;
-      ChangeSet changeSet = new ChangeSet();
-      for (String uri in uris) {
-        Source source = factory.forUri(uri);
-        changeSet.addedSource(source);
-      }
-      _analysisContext.applyChanges(changeSet);
     }
     return _analysisContext;
   }
 
   @override
   List<SdkLibrary> get sdkLibraries => LIBRARIES;
 
   @override
   String get sdkVersion => throw unimplemented;
 
@@ skipping 96 matching lines @@
   bool get isInternal => throw unimplemented;
 
   @override
   bool get isShared => throw unimplemented;
 
   @override
   bool get isVmLibrary => throw unimplemented;
 
   UnimplementedError get unimplemented => new UnimplementedError();
 }