Revert breaking change

pkg/analyzer/lib/src/plugin/engine_plugin.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 analyzer.src.plugin.engine_plugin;
 
+import 'package:analyzer/plugin/plugin.dart';
 import 'package:analyzer/plugin/task.dart';
 import 'package:analyzer/src/task/dart.dart';
 import 'package:analyzer/src/task/general.dart';
 import 'package:analyzer/task/model.dart';
-import 'package:plugin/plugin.dart';
 
 /**
  * A plugin that defines the extension points and extensions that are inherently
  * defined by the analysis engine.
  */
 class EnginePlugin implements Plugin {
   /**
    * The simple identifier of the extension point that allows plugins to
    * register new analysis tasks with the analysis engine.
    */
@@ skipping 69 matching lines @@
    * Validate the given extension by throwing an [ExtensionError] if it is not a
    * valid domain.
    */
   void _validateTaskExtension(Object extension) {
     if (extension is! TaskDescriptor) {
       String id = taskExtensionPoint.uniqueIdentifier;
       throw new ExtensionError('Extensions to $id must be a TaskDescriptor');
     }
   }
 }