Fix VM crash when parsing await throw

runtime/vm/ast_transformer.cc

Index: runtime/vm/ast_transformer.cc
===================================================================
--- runtime/vm/ast_transformer.cc	(revision 44136)
+++ runtime/vm/ast_transformer.cc	(working copy)
@@ -622,12 +622,10 @@
 
 
 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