First step in eliminating —enable_async flag

tests/language/vm/optimized_await_regress_test.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.
 
-// VMOptions=--enable_async --optimization-counter-threshold=10
+// VMOptions=--optimization-counter-threshold=10
 
 // This tests that captured parameters (by the async-closure) are
 // correctly treated in try-catch generated in the async function.
 // They must be skipped when generating sync-code in the optimized
 // try-block.
 
 import 'package:expect/expect.dart';
 
 import 'dart:async';
 
@@ skipping 16 matching lines @@
   if (a != "abc77") fail();
   return k;
 }
 
 
 main() {
   for (int i = 0; i < 20; i++) {
     foo(i).then((value) => Expect.equals(77, value));
   }
 }