Fix async_test problems.

pkg/compiler/lib/src/warnings.dart

 // Copyright (c) 2011, 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.
 
 part of dart2js;
 
 const DONT_KNOW_HOW_TO_FIX = "Computer says no!";
 
 /**
  * The messages in this file should meet the following guide lines:
@@ skipping 2126 matching lines @@
 }
 main() => new A();""",
 
 """
 class A {
   A();
   factory A.a() async* => new A();
 }
 main() => new A.a();"""]);
 
+  static const MessageKind ASYNC_MODIFIER_ON_SETTER =
+      const MessageKind(
+          "The modifier '#{modifier}' is not allowed on setters.",
+          options: const ['--enable-async'],
+          howToFix: "Try removing the '#{modifier}' modifier.",
+          examples: const ["""
+class A {
+  set foo(v) async {}
+}
+main() => new A().foo = 0;"""]);
+
   static const MessageKind YIELDING_MODIFIER_ON_ARROW_BODY =
       const MessageKind(
           "The modifier '#{modifier}' is not allowed on methods implemented "
           "using '=>'.",
           options: const ['--enable-async'],
           howToFix: "Try removing the '#{modifier}' modifier or implementing "
                     "the method body using a block: '{ ... }'.",
           examples: const ["main() sync* => null;", "main() async* => null;"]);
 
   // TODO(johnniwinther): Check for 'async' as identifier.
@@ skipping 282 matching lines @@
   static String convertToString(value) {
     if (value is ErrorToken) {
       // Shouldn't happen.
       return value.assertionMessage;
     } else if (value is Token) {
       value = value.value;
     }
     return '$value';
   }
 }