Fix analyzer warning by updating _MockSdkSource.

lib/src/dart_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 code_transformers.src.dart_sdk;
 
 import 'dart:io' show Directory;
 
 /// Note that the Analyzer has two versions of SdkAnalysisContext (and lots of
 /// other classes) with different signatures: can't mix the two.
@@ skipping 164 matching lines @@
   }
 }
 
 class _MockSdkSource implements UriAnnotatedSource {
   /// Absolute URI which this source can be imported from.
   final Uri uri;
   final String _contents;
 
   Source get source => this;
 
+  Source get librarySource => null;
+
   _MockSdkSource(this.uri, this._contents);
 
   bool exists() => true;
 
   int get hashCode => uri.hashCode;
 
   final int modificationStamp = 1;
 
   TimestampedData<String> get contents =>
       new TimestampedData(modificationStamp, _contents);
@@ skipping 63 matching lines @@
   'dart:async': '''
         class Future<T> {
           Future then(callback) {}
         class Stream<T> {}
   ''',
   'dart:html': '''
         library dart.html;
         class HtmlElement {}
         ''',
 };