Fix errors in dart2js/source_map_pub_build_validity_test

tests/compiler/dart2js/source_map_pub_build_validity_test.dart

 // Copyright (c) 2014, 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:io';
 
 import 'package:path/path.dart' as path;
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
 import 'source_map_validator_helper.dart';
 
 void main() {
   asyncTest(() async {
-    Directory tmpDir = createTempDir();
+    Directory tmpDir = await createTempDir();
     try {
       Directory sunflowerDir = new Directory.fromUri(
           Platform.script.resolve('../../../third_party/sunflower'));
 
       print("Copying '${sunflowerDir.path}' to '${tmpDir.path}'.");
       copyDirectory(sunflowerDir, tmpDir);
       String ext = Platform.isWindows ? '.bat' : '';
       String command = path.normalize(path.join(
           path.fromUri(Platform.script),
           '../../../../sdk/bin/pub${ext}'));
@@ skipping 10 matching lines @@
       ProcessResult buildResult = await Process.run(
           command, ['build','--mode=debug'], workingDirectory: tmpDir.path);
       print(buildResult.stdout);
       print(buildResult.stderr);
       Expect.equals(0, buildResult.exitCode, 'Unexpected exitCode from pub');
       validateSourceMap(new Uri.file(file, windows: Platform.isWindows));
       print("Deleting '${tmpDir.path}'.");
     } finally {
       tmpDir.deleteSync(recursive: true);
     }
-  }));
+  });
 }