Fix batch test as well.

tests/compiler/dart2js/dart2js_batch_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:async';
 import 'dart:convert';
 import 'dart:io';
 
 import 'package:path/path.dart' as path;
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
 
+import 'launch_helper.dart' show dart2JsCommand;
+
 var tmpDir;
 
 copyDirectory(Directory sourceDir, Directory destinationDir) {
   sourceDir.listSync().forEach((FileSystemEntity element) {
     String newPath = path.join(destinationDir.path,
                                path.basename(element.path));
     if (element is File) {
       element.copySync(newPath);
     } else if (element is Directory) {
       Directory newDestinationDir = new Directory(newPath);
@@ skipping 21 matching lines @@
     copyDirectory(sunflowerDir, tmpDir);
   });
 }
 
 void cleanUp() {
   print("Deleting '${tmpDir.path}'.");
   tmpDir.deleteSync(recursive: true);
 }
 
 Future launchDart2Js(_) {
-  String ext = Platform.isWindows ? '.bat' : '';
-  String command =
-      path.normalize(path.join(path.fromUri(Platform.script),
-                    '../../../../sdk/bin/dart2js${ext}'));
-  print("Running '$command --batch' from '${tmpDir}'.");
-  return Process.start(command, ['--batch'], workingDirectory: tmpDir.path);
+  return Process.start(
+      // Use an absolute path because we are changing the cwd below.
+      path.fromUri(Uri.base.resolve(Platform.executable)),
+      dart2JsCommand(['--batch']),
+      workingDirectory: tmpDir.path);
 }
 
 Future runTests(Process process) {
   String inFile = path.join(tmpDir.path, 'web/sunflower.dart');
   String outFile = path.join(tmpDir.path, 'out.js');
   String outFile2 = path.join(tmpDir.path, 'out2.js');
 
   process.stdin.writeln('--out="$outFile" "$inFile"');
   process.stdin.writeln('--out="$outFile2" "$inFile"');
   process.stdin.writeln('too many arguments');
@@ skipping 17 matching lines @@
 
 void main() {
   var tmpDir;
   asyncTest(() {
     return setup()
         .then(launchDart2Js)
         .then(runTests)
         .whenComplete(cleanUp);
   });
 }