Don't add sources to DartSdk contexts.

pkg/analyzer/test/src/mock_sdk.dart

Index: pkg/analyzer/test/src/mock_sdk.dart
diff --git a/pkg/analyzer/test/src/mock_sdk.dart b/pkg/analyzer/test/src/mock_sdk.dart
index 1ff681faa6962347cdade383f99e5aab0a369839..bfef108465b73b07f437629c4d23e3766791f4d6 100644
--- a/pkg/analyzer/test/src/mock_sdk.dart
+++ b/pkg/analyzer/test/src/mock_sdk.dart
@@ -209,12 +209,6 @@ class HtmlElement {}
       _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;
   }