Call asyncEnd only on success.

tests/compiler/dart2js/bad_loop_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.
 
 import 'package:expect/expect.dart';
 import 'memory_source_file_helper.dart';
 import "package:async_helper/async_helper.dart";
 
 import '../../../sdk/lib/_internal/compiler/compiler.dart'
        show Diagnostic;
@@ skipping 19 matching lines @@
       throw 'unexpected diagnostic $kind: $message';
     }
   }
 
   Compiler compiler = new Compiler(provider.readStringFromUri,
                                    (name, extension) => null,
                                    diagnosticHandler,
                                    libraryRoot,
                                    packageRoot,
                                    ['--analyze-only']);
-  asyncStart();
-  compiler.run(Uri.parse('memory:main.dart')).then((_) {
+  asyncTest(() => compiler.run(Uri.parse('memory:main.dart')).then((_) {
     Expect.isTrue(compiler.compilationFailed);
     Expect.equals(5, errorCount);
     Expect.equals(1, warningCount);
-  }).whenComplete(() => asyncEnd());
+  }));
 }
 
 const Map MEMORY_SOURCE_FILES = const {
   'main.dart': """
 main() {
   for (var x, y in []) {
   }
 
   for (var x = 10 in []) {
   }
 
   for (x.y in []) { // Also causes a warning "x unresolved".
   }
 
   for ((){}() in []) {
   }
 
   for (1 in []) {
   }
 }
 """
 };