Make dart2js unittests async.

tests/compiler/dart2js/minify_many_locals_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.
 // 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() {
   var buffer = new StringBuffer();
   buffer.write("var foo(");
   for (int i = 0; i < 2000; i++) {
     buffer.write("x$i, ");
   }
   buffer.write("x) { int i = ");
   for (int i = 0; i < 2000; i++) {
     buffer.write("x$i+");
   }
   buffer.write("2000; return i; }");
-  var generated = compile(buffer.toString(), entry: 'foo', minify: true);
-  RegExp re = new RegExp(r"\(a,b,c");
-  Expect.isTrue(re.hasMatch(generated));
+  String code = buffer.toString();
 
-  re = new RegExp(r"x,y,z,a0,a1,a2");
-  Expect.isTrue(re.hasMatch(generated));
+  asyncTest(() => compile(code, entry: 'foo', minify: true)
+      .then((String generated) {
+    RegExp re = new RegExp(r"\(a,b,c");
+    Expect.isTrue(re.hasMatch(generated));
 
-  re = new RegExp(r"y,z,a0,a1,a2,a3,a4,a5,a6");
-  Expect.isTrue(re.hasMatch(generated));
+    re = new RegExp(r"x,y,z,a0,a1,a2");
+    Expect.isTrue(re.hasMatch(generated));
 
-  re = new RegExp(r"g8,g9,h0,h1");
-  Expect.isTrue(re.hasMatch(generated));
+    re = new RegExp(r"y,z,a0,a1,a2,a3,a4,a5,a6");
+    Expect.isTrue(re.hasMatch(generated));
 
-  re = new RegExp(r"z8,z9,aa0,aa1,aa2");
-  Expect.isTrue(re.hasMatch(generated));
+    re = new RegExp(r"g8,g9,h0,h1");
+    Expect.isTrue(re.hasMatch(generated));
 
-  re = new RegExp(r"aa9,ab0,ab1");
-  Expect.isTrue(re.hasMatch(generated));
+    re = new RegExp(r"z8,z9,aa0,aa1,aa2");
+    Expect.isTrue(re.hasMatch(generated));
 
-  re = new RegExp(r"az9,ba0,ba1");
-  Expect.isTrue(re.hasMatch(generated));
+    re = new RegExp(r"aa9,ab0,ab1");
+    Expect.isTrue(re.hasMatch(generated));
+
+    re = new RegExp(r"az9,ba0,ba1");
+    Expect.isTrue(re.hasMatch(generated));
+  }));
 }