gracefully degrade with response to client if fail to access client specified dart sdk

pkg/analysis_server/test/mocks.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 mocks;
 
 import 'dart:async';
-import 'dart:convert';
 import 'dart:io';
 
 /**
  * A mock [WebSocket] for testing.
  */
 class MockSocket<T> implements WebSocket {
 
-  final JsonEncoder jsonEncoder = const JsonEncoder(null);
-
-  final JsonDecoder jsonDecoder = const JsonDecoder(null);
-
   StreamController controller = new StreamController();
   MockSocket twin;
   Stream stream;
 
   factory MockSocket.pair() {
     MockSocket socket1 = new MockSocket();
     MockSocket socket2 = new MockSocket();
     socket1.twin = socket2;
     socket2.twin = socket1;
     socket1.stream = socket2.controller.stream;
@@ skipping 13 matching lines @@
       .then((_) => twin.controller.close());
 
   StreamSubscription<T> listen(void onData(T event),
                      { Function onError, void onDone(), bool cancelOnError}) =>
     stream.listen(onData, onError: onError, onDone: onDone,
         cancelOnError: cancelOnError);
 
   Stream<T> where(bool test(T)) => stream.where(test);
 
   noSuchMethod(Invocation invocation) => super.noSuchMethod(invocation);
-}
+}