fix invocation completion test

pkg/analysis_server/test/mock_sdk.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 testing.mock_sdk;
 
 import 'package:analyzer/file_system/file_system.dart' as resource;
 import 'package:analyzer/file_system/memory_file_system.dart' as resource;
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/sdk.dart';
 import 'package:analyzer/src/generated/source.dart';
 
 
 class MockSdk implements DartSdk {
   static const _MockSdkLibrary LIB_CORE =
       const _MockSdkLibrary('dart:core', '/lib/core/core.dart', '''
 library dart.core;
 
 import 'dart:async';
 
 class Object {
   bool operator ==(other) => identical(this, other);
   String toString() => 'a string';
+  int get hashCode => 0;
 }
 
 class Function {}
 class StackTrace {}
 class Symbol {}
 class Type {}
 
 abstract class Comparable<T> {
   int compareTo(T other);
 }
@@ skipping 223 matching lines @@
   bool get isInternal => throw unimplemented;
 
   @override
   bool get isShared => throw unimplemented;
 
   @override
   bool get isVmLibrary => throw unimplemented;
 
   UnimplementedError get unimplemented => new UnimplementedError();
 }