Leave "error" key out of response JSON when there is no error.

pkg/analysis_server/test/protocol_test.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 test.protocol;
 
 import 'dart:convert';
 
 import 'package:analysis_server/src/protocol.dart';
 import 'package:unittest/unittest.dart';
@@ skipping 439 matching lines @@
 
   @runTest
   static void setResult() {
     String resultName = 'name';
     String resultValue = 'value';
     Response response = new Response('0');
     response.setResult(resultName, resultValue);
     expect(response.getResult(resultName), same(resultValue));
     expect(response.toJson(), equals({
       Response.ID: '0',
-      Response.ERROR: null,
       Response.RESULT: {
         resultName: resultValue
       }
     }));
   }
 
   @runTest
   static void fromJson() {
     Response original = new Response('myId');
     Response response = new Response.fromJson(original.toJson());
@@ skipping 17 matching lines @@
     original.setResult('foo', 'bar');
     Response response = new Response.fromJson(original.toJson());
     expect(response.id, equals('myId'));
     Map<String, Object> result = response.result;
     expect(result.length, equals(1));
     expect(result['foo'], equals('bar'));
   }
 }
 
 Matcher _throwsRequestFailure = throwsA(new isInstanceOf<RequestFailure>());