Fix VM crash when parsing await throw

runtime/vm/ast_transformer.cc

 // 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.
 
 #include "vm/ast_transformer.h"
 
 #include "vm/object_store.h"
 #include "vm/parser.h"
 #include "vm/thread.h"
 
@@ skipping 604 matching lines @@
   // TODO(mlippautz): Check initializers and their temps.
   LetNode* result = new(Z) LetNode(node->token_pos());
   for (intptr_t i = 0; i < node->nodes().length(); i++) {
     result->AddNode(Transform(node->nodes()[i]));
   }
   result_ = result;
 }
 
 
 void AwaitTransformer::VisitThrowNode(ThrowNode* node) {
-  // TODO(mlippautz): Check if relevant.
   AstNode* new_exception = Transform(node->exception());
-  AstNode* new_stacktrace = Transform(node->stacktrace());
   result_ = new(Z) ThrowNode(node->token_pos(),
                              new_exception,
-                             new_stacktrace);
+                             node->stacktrace());
 }
 
 }  // namespace dart