Make source be a target

pkg/analyzer/test/generated/test_support.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.
 
 // This code was auto-generated, is not intended to be edited, and is subject to
 // significant change. Please see the README file for more information.
 
 library engine.test_support;
 
 import 'dart:collection';
@@ skipping 542 matching lines @@
     infoCount++;
   }
 
   @override
   void logInformation2(String message, Object exception) {
     infoCount++;
   }
 }
 
 
-class TestSource implements Source {
+class TestSource extends Source {
   String _name;
   String _contents;
   int modificationStamp = 0;
   bool exists2 = true;
 
   /**
    * A flag indicating whether an exception should be generated when an attempt
    * is made to access the contents of this source.
    */
   bool generateExceptionOnRead = false;
@@ skipping 46 matching lines @@
     throw new UnsupportedOperationException();
   }
   Uri resolveRelativeUri(Uri uri) {
     return new Uri(scheme: 'file', path: _name).resolveUri(uri);
   }
   void setContents(String value) {
     modificationStamp = new DateTime.now().millisecondsSinceEpoch;
     _contents = value;
   }
 }