Call asyncEnd only on success.

tests/compiler/dart2js/class_codegen2_test.dart

 // Copyright (c) 2012, 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 'dart:async';
 import "package:expect/expect.dart";
 import "package:async_helper/async_helper.dart";
 import 'compiler_helper.dart';
 import 'parser_helper.dart';
@@ skipping 72 matching lines @@
 main() {
   foo(new A().foo);
 }
 """;
 
 main() {
   // At some point Dart2js generated bad object literals with dangling commas:
   // { a: true, }. Make sure this doesn't happen again.
   RegExp danglingComma = new RegExp(r',[ \n]*}');
 
-  asyncStart();
-  Future.forEach([TEST_ONE, TEST_TWO, TEST_THREE, TEST_FOUR], (test) {
+  asyncTest(() => Future.forEach([TEST_ONE, TEST_TWO, TEST_THREE, TEST_FOUR],
+      (test) {
     return compileAll(test).then((generated) {
       Expect.isFalse(danglingComma.hasMatch(generated));
     });
-  }).whenComplete(() => asyncEnd());
+  }));
 }