Fix static warning in a test.

tests/language/vm/optimized_guarded_field_isolates_test.dart

 // Copyright (c) 2013, 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=--optimization_counter_threshold=100
 
 // Test field type tracking and field list-length tracking in the presence of
 // multiple isolates.
 
 import "dart:isolate";
+import "dart:async";
 import "package:expect/expect.dart";
 
 class A {
   A(this.a);
   var a;
 }
 
 class B extends A {
   B(a, this.b) : super(a) { }
 
@@ skipping 40 matching lines @@
   item.then((C value) {
       Expect.throws(() => test_c(value), (e) => e is RangeError);
       receive_port.close();
   });
 }
 
 main() {
   test_field_type();
   test_list_length();
 }