Make source be a target

pkg/analyzer/lib/file_system/memory_file_system.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 memory_file_system;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:analyzer/src/generated/engine.dart' show TimestampedData;
@@ skipping 194 matching lines @@
   @override
   bool isOrContains(String path) {
     return path == this.path;
   }
 }
 
 
 /**
  * An in-memory implementation of [Source].
  */
-class _MemoryFileSource implements Source {
+class _MemoryFileSource extends Source {
   final _MemoryFile _file;
 
   final Uri uri;
 
   _MemoryFileSource(this._file, this.uri);
 
   @override
   TimestampedData<String> get contents {
     return new TimestampedData<String>(modificationStamp, _file._content);
   }
@@ skipping 150 matching lines @@
   bool operator ==(other) {
     if (runtimeType != other.runtimeType) {
       return false;
     }
     return path == other.path;
   }
 
   @override
   String toString() => path;
 }