Unify the server and plugin versions of the generators

pkg/analyzer_plugin/tool/spec/api.dart

-// Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
+// 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.
 
 /**
  * Data structures representing an API definition, and visitor base classes
  * for visiting those data structures.
  */
 import 'dart:collection';
 
 import 'package:html/dom.dart' as dom;
@@ skipping 503 matching lines @@
    * The field that is used to disambiguate this union
    */
   final String field;
 
   TypeUnion(this.choices, this.field, dom.Element html, {bool experimental})
       : super(html, experimental, false);
 
   @override
   accept(ApiVisitor visitor) => visitor.visitTypeUnion(this);
 }