Issue 24298. Fix for 'async' function body conversion.

pkg/analyzer/lib/src/generated/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 engine.ast;
 
 import 'dart:collection';
 
 import 'element.dart';
 import 'engine.dart' show AnalysisEngine;
@@ skipping 3459 matching lines @@
    * Initialize a newly created function body consisting of a block of
    * statements. The [keyword] can be `null` if there is no keyword specified
    * for the block. The [star] can be `null` if there is no star following the
    * keyword (and must be `null` if there is no keyword).
    */
   BlockFunctionBody(this.keyword, this.star, Block block) {
     _block = _becomeParentOf(block);
   }
 
   @override
-  Token get beginToken => _block.beginToken;
+  Token get beginToken {
+    if (keyword != null) {
+      return keyword;
+    }
+    return _block.beginToken;
+  }
 
   /**
    * Return the block representing the body of the function.
    */
   Block get block => _block;
 
   /**
    * Set the block representing the body of the function to the given [block].
    */
   void set block(Block block) {
@@ skipping 16709 matching lines @@
   }
 
   @override
   accept(AstVisitor visitor) => visitor.visitYieldStatement(this);
 
   @override
   void visitChildren(AstVisitor visitor) {
     _safelyVisitChild(_expression, visitor);
   }
 }