Make source be a target

pkg/analyzer/lib/src/string_source.dart

 // Copyright (c) 2013, 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.string_source;
 
 import 'generated/engine.dart' show TimestampedData;
 import 'generated/source.dart';
 
 /// An implementation of [Source] that's based on an in-memory Dart string.
-class StringSource implements Source {
+class StringSource extends Source {
   final String _contents;
   final String fullName;
   final int modificationStamp;
 
   StringSource(this._contents, this.fullName)
       : modificationStamp = new DateTime.now().millisecondsSinceEpoch;
 
   TimestampedData<String> get contents =>
       new TimestampedData(modificationStamp, _contents);
 
@@ skipping 19 matching lines @@
       return ssObject._contents == _contents && ssObject.fullName == fullName;
     }
     return false;
   }
 
   bool exists() => true;
 
   Uri resolveRelativeUri(Uri relativeUri) =>
       throw new UnsupportedError("StringSource doesn't support resolveRelative.");
 }