Async closures are not optimizable for now.

runtime/vm/object.cc

 // Copyright (c) 2012, 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/object.h"
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "vm/assembler.h"
 #include "vm/cpu.h"
@@ skipping 5507 matching lines @@
 }
 
 
 void Function::set_is_external(bool value) const {
   set_kind_tag(ExternalBit::update(value, raw_ptr()->kind_tag_));
 }
 
 
 void Function::set_is_async_closure(bool value) const {
   set_kind_tag(AsyncClosureBit::update(value, raw_ptr()->kind_tag_));
+  set_is_optimizable(false);
 }
 
 
 void Function::set_token_pos(intptr_t value) const {
   ASSERT(value >= 0);
   raw_ptr()->token_pos_ = value;
 }
 
 
 void Function::set_kind_tag(intptr_t value) const {
@@ skipping 13927 matching lines @@
   return tag_label.ToCString();
 }
 
 
 void UserTag::PrintJSONImpl(JSONStream* stream, bool ref) const {
   Instance::PrintJSONImpl(stream, ref);
 }
 
 
 }  // namespace dart