Remove the old task model

pkg/analyzer/test/src/context/abstract_context.dart

 // Copyright (c) 2015, 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.src.task.abstract_context_test;
 
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/file_system/memory_file_system.dart';
 import 'package:analyzer/src/context/cache.dart';
 import 'package:analyzer/src/context/context.dart';
 import 'package:analyzer/src/generated/element.dart';
-import 'package:analyzer/src/generated/engine.dart'
-    hide AnalysisCache, AnalysisContextImpl, AnalysisTask;
+import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/sdk.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:analyzer/src/task/driver.dart';
 import 'package:analyzer/task/model.dart';
 import 'package:plugin/manager.dart';
 import 'package:plugin/plugin.dart';
 import 'package:unittest/unittest.dart';
 
 import 'mock_sdk.dart';
 
@@ skipping 108 matching lines @@
     plugins.add(AnalysisEngine.instance.optionsPlugin);
 
     ExtensionManager manager = new ExtensionManager();
     manager.processPlugins(plugins);
 
     prepareAnalysisContext();
   }
 
   void tearDown() {}
 }