Call asyncEnd only on success.

tests/compiler/dart2js/async_compiler_input_provider_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.
 
 import "dart:async";
 import "dart:io";
 
 import "package:expect/expect.dart";
 import "package:async_helper/async_helper.dart";
 
@@ skipping 28 matching lines @@
 }
 
 main() {
   var entrypoint = Uri.parse("file:///main.dart");
 
   // Find the path to sdk/ in the repo relative to this script.
   Uri script = currentDirectory.resolve(nativeToUriPath(Platform.script));
   Uri libraryRoot = script.resolve('../../../sdk/');
   Uri packageRoot = script.resolve('./packages/');
 
-  asyncStart();
-  compiler.compile(entrypoint, libraryRoot, packageRoot,
+  asyncTest(() => compiler.compile(entrypoint, libraryRoot, packageRoot,
       provideInput, handleDiagnostic, []).then((code) {
     Expect.isNotNull(code);
-  }).whenComplete(() => asyncEnd());
+  }));
 }
 
 void handleDiagnostic(Uri uri, int begin, int end, String message,
                       compiler.Diagnostic kind) {
   print(message);
   if (kind != compiler.Diagnostic.VERBOSE_INFO) {
     throw 'Unexpected diagnostic kind $kind';
   }
 }