Don't add sources to DartSdk contexts.

pkg/analyzer/test/src/context/mock_sdk.dart

Index: pkg/analyzer/test/src/context/mock_sdk.dart
diff --git a/pkg/analyzer/test/src/context/mock_sdk.dart b/pkg/analyzer/test/src/context/mock_sdk.dart
index 5cf93dd168055278b122b2239f63609f0d0bb8e4..b67c4a56cfa09c13861c66c4be6466134d4b6a0f 100644
--- a/pkg/analyzer/test/src/context/mock_sdk.dart
+++ b/pkg/analyzer/test/src/context/mock_sdk.dart
@@ -228,12 +228,6 @@ class HtmlElement {}
       _analysisContext = new _SdkAnalysisContext(this);
       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;
   }