Issue 26051. Fix for MethodDeclartion.beginToken for external.

pkg/analyzer/lib/src/dart/ast/ast.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.
 
 library analyzer.src.dart.ast.ast;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
@@ skipping 7058 matching lines @@
    */
   @override
   ExecutableElement get element =>
       _name != null ? (_name.staticElement as ExecutableElement) : null;
 
   @override
   Token get endToken => _body.endToken;
 
   @override
   Token get firstTokenAfterCommentAndMetadata {
-    if (modifierKeyword != null) {
+    if (externalKeyword != null) {
+      return externalKeyword;
+    } else if (modifierKeyword != null) {
       return modifierKeyword;
     } else if (_returnType != null) {
       return _returnType.beginToken;
     } else if (propertyKeyword != null) {
       return propertyKeyword;
     } else if (operatorKeyword != null) {
       return operatorKeyword;
     }
     return _name.beginToken;
   }
@@ skipping 3984 matching lines @@
 
   @override
   dynamic/*=E*/ accept/*<E>*/(AstVisitor/*<E>*/ visitor) =>
       visitor.visitYieldStatement(this);
 
   @override
   void visitChildren(AstVisitor visitor) {
     _expression?.accept(visitor);
   }
 }