Eliminate dependencies on pkg/front_end.

pkg/analyzer_plugin/test/plugin/mocks.dart

 // Copyright (c) 2017, 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.
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/src/dart/analysis/driver.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:analyzer_plugin/channel/channel.dart';
 import 'package:analyzer_plugin/plugin/plugin.dart';
 import 'package:analyzer_plugin/protocol/protocol.dart';
 import 'package:analyzer_plugin/protocol/protocol_generated.dart';
 import 'package:analyzer_plugin/src/protocol/protocol_internal.dart';
-import 'package:front_end/src/base/timestamped_data.dart';
+import 'package:analyzer/src/generated/timestamped_data.dart';
 import 'package:test/test.dart';
 
 class MockAnalysisDriver extends AnalysisDriver {
   @override
   Set<String> addedFiles = new HashSet<String>();
 
   MockAnalysisDriver()
       : super(new AnalysisDriverScheduler(null), null, null, null, null, null,
             new SourceFactory([]), new AnalysisOptionsImpl());
 
@@ skipping 129 matching lines @@
 
   @override
   Uri get uri => Uri.parse('package:test/test.dart');
 
   @override
   UriKind get uriKind => UriKind.PACKAGE_URI;
 
   @override
   bool exists() => true;
 }