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

pkg/analysis_server/lib/src/protocol.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 protocol;
 
 import 'dart:convert' show JsonDecoder;
 
 /**
  * Instances of the class [Request] represent a request that was received.
@@ skipping 358 matching lines @@
   /**
    * An error code indicating one or more invalid parameters.
    */
   static const int CODE_INVALID_PARAMS = -32602;
 
   /**
    * An error code indicating an internal error.
    */
   static const int CODE_INTERNAL_ERROR = -32603;
 
+  /**
+   * An error code indicating a problem using the specified Dart SDK.
+   */
+  static const int CODE_SDK_ERROR = -32603;
+
   /*
    * In addition, codes -32000 to -32099 indicate a server error. They are
    * reserved for implementation-defined server-errors.
    */
 
   /**
    * The code that uniquely identifies the error that occurred.
    */
   final int code;
 
@@ skipping 190 matching lines @@
   /**
    * The response to be returned as a result of the failure.
    */
   final Response response;
 
   /**
    * Initialize a newly created exception to return the given reponse.
    */
   RequestFailure(this.response);
 }