Gracefully degrade if no source for completion

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:collection';
 import 'dart:convert';
 
 part 'generated_protocol.dart';
@@ skipping 808 matching lines @@
               "Unanalyzed files cannot be a priority: '$fileNames'"));
 
   /**
    * Initialize a newly created instance to represent an error condition caused
    * by a [request] that cannot be handled by any known handlers.
    */
   Response.unknownRequest(Request request) : this(request.id,
           error: new RequestError(
               RequestErrorCode.UNKNOWN_REQUEST, 'Unknown request'));
 
+  /**
+   * Initialize a newly created instance to represent an error condition caused
+   * by a [request] referencing a source that does not exist.
+   */
+  Response.unknownSource(Request request) : this(request.id,
+          error: new RequestError(
+              RequestErrorCode.UNKNOWN_SOURCE, 'Unknown source'));
+
   Response.unsupportedFeature(String requestId, String message) : this(
           requestId,
           error: new RequestError(
               RequestErrorCode.UNSUPPORTED_FEATURE, message));
 
   /**
    * Return a table representing the structure of the Json object that will be
    * sent to the client to represent this response.
    */
   Map<String, Object> toJson() {
@@ skipping 46 matching lines @@
     hash = 0x1fffffff & (hash + ((0x03ffffff & hash) << 3));
     hash = hash ^ (hash >> 11);
     return 0x1fffffff & (hash + ((0x00003fff & hash) << 15));
   }
 
   static int hash2(a, b) => finish(combine(combine(0, a), b));
 
   static int hash4(a, b, c, d) =>
       finish(combine(combine(combine(combine(0, a), b), c), d));
 }