Add isExternal accessor to element model

pkg/analyzer/lib/src/generated/element_handle.dart

 // 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.
 
 // This code was auto-generated, is not intended to be edited, and is subject to
 // significant change. Please see the README file for more information.
 
 library engine.element_handle;
 
 import 'ast.dart';
@@ skipping 503 matching lines @@
   @override
   List<FunctionElement> get functions => actualElement.functions;
 
   @override
   bool get isAbstract => actualElement.isAbstract;
 
   @override
   bool get isAsynchronous => actualElement.isAsynchronous;
 
   @override
+  bool get isExternal => actualElement.isExternal;
+
+  @override
   bool get isGenerator => actualElement.isGenerator;
 
   @override
   bool get isOperator => actualElement.isOperator;
 
   @override
   bool get isStatic => actualElement.isStatic;
 
   @override
   bool get isSynchronous => actualElement.isSynchronous;
@@ skipping 568 matching lines @@
   DartType get type => actualElement.type;
 }
 /**
  * TODO(scheglov) invalid implementation
  */
 class WeakReference<T> {
   final T value;
   WeakReference(this.value);
   T get() => value;
 }