Fix parent pointer issue in async rewriter.

lib/ast.dart

 // Copyright (c) 2016, 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.
 
 /// -----------------------------------------------------------------------
 ///                           ERROR HANDLING
 /// -----------------------------------------------------------------------
 ///
 /// As a rule of thumb, errors that can be detected statically are handled by
 /// the frontend, typically by translating the erroneous code into a 'throw' or
@@ skipping 2437 matching lines @@
 
   accept(StatementVisitor v) => v.visitBlock(this);
 
   visitChildren(Visitor v) {
     visitList(statements, v);
   }
 
   transformChildren(Transformer v) {
     transformList(statements, v, this);
   }
+
+  void addStatement(Statement node) {
+    statements.add(node);
+    node.parent = this;
+  }
 }
 
 class EmptyStatement extends Statement {
   accept(StatementVisitor v) => v.visitEmptyStatement(this);
 
   visitChildren(Visitor v) {}
   transformChildren(Transformer v) {}
 }
 
 class AssertStatement extends Statement {
@@ skipping 1133 matching lines @@
 
   @override
   defaultTreeNode(TreeNode node) {
     if (node == child) {
       return replacement;
     } else {
       return node;
     }
   }
 }