Make dart2js unittests async.

tests/compiler/dart2js/declare_once_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.
 // Test that parameters keep their names in the output.
 
-import "package:expect/expect.dart";
+import 'package:async_helper/async_helper.dart';
+import 'package:expect/expect.dart';
 import 'compiler_helper.dart';
 
 main() {
   // For a function with only one variable we declare it inline for more
   // compactness.  Test that we don't also declare it at the start of the
   // method.
-  var generated = compile(
+  asyncTest(() => compile(
       'final List a = const ["bar", "baz"];'
       'int foo() {'
       '  for (int i = 0; i < a.length; i++) {'
       '    print(a[i]);'
       '  }'
       '}',
-      entry: 'foo', minify: false);
-  RegExp re = new RegExp(r"var ");
-  Expect.isTrue(re.hasMatch(generated));
-  print(generated);
-  Expect.equals(1, re.allMatches(generated).length);
+      entry: 'foo', minify: false).then((String generated) {
+    RegExp re = new RegExp(r"var ");
+    Expect.isTrue(re.hasMatch(generated));
+    print(generated);
+    Expect.equals(1, re.allMatches(generated).length);
+  }));
 }