Break up another large file

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/error/error.dart' show AnalysisError;
 import 'package:analyzer/plugin/task.dart';
 import 'package:analyzer/src/generated/engine.dart'
     show InternalAnalysisContext;
-import 'package:analyzer/src/generated/error.dart' show AnalysisError;
 import 'package:analyzer/src/task/dart.dart';
 import 'package:analyzer/src/task/dart_work_manager.dart';
 import 'package:analyzer/src/task/general.dart';
 import 'package:analyzer/src/task/html.dart';
 import 'package:analyzer/src/task/html_work_manager.dart';
 import 'package:analyzer/src/task/options_work_manager.dart';
 import 'package:analyzer/src/task/yaml.dart';
 import 'package:analyzer/task/model.dart';
 import 'package:plugin/plugin.dart';
 
@@ skipping 248 matching lines @@
  * passed to the extension manager to populate it.
  *
  * This annotation is not used at runtime; it is used to aid in static analysis
  * of the task model during development.
  */
 class ExtensionPointId {
   final String extensionPointId;
 
   const ExtensionPointId(this.extensionPointId);
 }