Fix real_version_test on Windows.

sdk/lib/_internal/pub/test/real_version_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.
 
 library pub_tests;
 
 import 'dart:io';
 
 import 'package:path/path.dart' as path;
 import 'package:scheduled_test/scheduled_process.dart';
 import 'package:scheduled_test/scheduled_test.dart';
 
 import '../lib/src/exit_codes.dart' as exit_codes;
-import '../lib/src/sdk.dart' as sdk;
 import 'test_pub.dart';
 
 main() {
   initConfig();
 
   // This test is a bit funny.
   //
   // Pub parses the "version" file that gets generated and shipped with the SDK.
   // We want to make sure that the actual version file that gets created is
   // also one pub can parse. If this test fails, it means the version file's
   // format has changed in a way pub didn't expect.
   //
   // Note that this test expects to be invoked from a Dart executable that is
   // in the built SDK's "bin" directory. Note also that this invokes pub from
   // the built SDK directory, and not the live pub code directly in the repo.
   integration('parse the real SDK "version" file', () {
-    // Get the path to the pub binary in the SDK.
-    var pubPath = path.join(sdk.rootDirectory, 'bin',
+    // Get the path to the pub binary in the SDK. Note that we can't use
+    // sdk.rootDirectory here because that assumes the entrypoint Dart script
+    // being run is pub itself. Here, the entrypoint is this test file.
+    var pubPath = path.join(path.dirname(Platform.executable),
         Platform.operatingSystem == "windows" ? "pub.bat" : "pub");
 
     var pub = new ScheduledProcess.start(pubPath, ['version']);
     pub.stdout.expect(startsWith("Pub"));
     pub.shouldExit(exit_codes.SUCCESS);
   });
 }