Fix static typing problem in analysis_server tests.

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:io';
 
 import 'package:analysis_server/plugin/protocol/protocol.dart'
@@ skipping 231 matching lines @@
     if (_closed) {
       return;
     }
     responsesReceived.add(response);
     // Wrap send response in future to simulate WebSocket.
     new Future(() => responseController.add(response));
   }
 
   Future<Response> waitForResponse(Request request) {
     String id = request.id;
-    return new Future<Response>(() =>
-        responseController.stream.firstWhere((response) => response.id == id));
+    return new Future<Response>(() => responseController.stream
+        .firstWhere((response) => response.id == id) as Response);
   }
 }
 
 /**
  * A mock [ServerOperation] for testing [AnalysisServer].
  */
 class MockServerOperation implements PerformAnalysisOperation {
   final ServerOperationPriority priority;
   final MockServerOperationPerformFunction _perform;
 
@@ skipping 167 matching lines @@
     }
     return mismatchDescription;
   }
 
   @override
   bool matches(item, Map matchState) {
     Response response = item;
     return response != null && response.id == _id && response.error == null;
   }
 }