Make dart2js unittests async.

tests/compiler/dart2js/string_escapes_test.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.
 
-import "package:expect/expect.dart";
+import 'dart:async';
+import 'package:async_helper/async_helper.dart';
+import 'package:expect/expect.dart';
 import 'compiler_helper.dart';
 
 // Test that the compiler handles string literals containing line terminators.
 
-String compileExpression(String expression) {
+Future<String> compileExpression(String expression) {
   var source = "foo() { return $expression; }";
   return compile(source, entry: "foo");
 }
 
 main() {
-  String generated = compileExpression("''' \n\r\u2028\u2029'''");
-  Expect.isTrue(generated.contains(r"\n\r\u2028\u2029"));
-
-  generated = compileExpression("r''' \n\r\u2028\u2029'''");
-  Expect.isTrue(generated.contains(r"\n\r\u2028\u2029"));
-
-  generated = compileExpression("'\u2028\u2029'");
-  Expect.isTrue(generated.contains(r"\u2028\u2029"));
+  asyncTest(() => Future.wait([
+    compileExpression("''' \n\r\u2028\u2029'''").then((String generated) {
+      Expect.isTrue(generated.contains(r"\n\r\u2028\u2029"));
+    }),
+    compileExpression("r''' \n\r\u2028\u2029'''").then((String generated) {
+      Expect.isTrue(generated.contains(r"\n\r\u2028\u2029"));
+    }),
+    compileExpression("'\u2028\u2029'").then((String generated) {
+      Expect.isTrue(generated.contains(r"\u2028\u2029"));
+    }),
+  ]));
 }