Make source be a target

pkg/analyzer/lib/src/generated/source_io.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.source.io;
 
 import 'engine.dart';
@@ skipping 68 matching lines @@
         path.codeUnitAt(path.length - 1) == JavaFile.separatorChar) {
       return path;
     }
     return "$path${JavaFile.separator}";
   }
 }
 
 /**
  * Instances of the class `FileBasedSource` implement a source that represents a file.
  */
-class FileBasedSource implements Source {
-
+class FileBasedSource extends Source {
   /**
    * A function that changes the way that files are read off of disk.
    */
   static Function fileReadMode = (String s) => s;
 
   /**
    * The URI from which this source was originally derived.
    */
   final Uri uri;
 
@@ skipping 438 matching lines @@
   }
 
   /**
    * Return `true` if the given URI is a `file` URI.
    *
    * @param uri the URI being tested
    * @return `true` if the given URI is a `file` URI
    */
   static bool isFileUri(Uri uri) => uri.scheme == FILE_SCHEME;
 }