Remove uses of Path class from tests/standalone.

tests/standalone/io/skipping_dart2js_compilations_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.
 
 /*
  * This test makes sure that the "skipping Dart2Js compilations if the output is
  * already up to date" feature does work as it should.
  * Therefore this test ensures that compilations are only skipped if the last
  * modified date of the output of a dart2js compilation is newer than
  *   - the the dart application to compile (including it's dependencies)
  *   - the dart2js snapshot
  * Furtheremore it ensure that a compilations is not skipped if any of the
  * necessary files could not be found (dart2js snapshots, previous dart2js
  * output (+deps file), dart application)
  */
 
 import 'package:expect/expect.dart';
+import 'package:path/path.dart';
 import 'dart:async';
 import 'dart:io';
 import '../../../tools/testing/dart/test_suite.dart' as suite;
 import '../../../tools/testing/dart/test_runner.dart' as runner;
 import '../../../tools/testing/dart/test_options.dart' as options;
 import '../../../tools/testing/dart/status_file_parser.dart' as status;
 
 /**
  * This class is reponsible for setting up the files necessary for this test
  * as well as touching a file.
@@ skipping 111 matching lines @@
           .existsSync());
     } else {
       Expect.isFalse(new File(fileUtils.scriptOutputPath.toNativePath())
           .existsSync());
     }
   }
 }
 
 runner.TestCase makeTestCase(String testName, FileUtils fileUtils) {
   var config = new options.TestOptionsParser().parse(['--timeout', '2'])[0];
-  var scriptDirPath = new Path(Platform.script).directoryPath;
-  var createFileScript = scriptDirPath.
-      append('skipping_dart2js_compilations_helper.dart').toNativePath();
+  var createFileScript = join(dirname(Platform.script),
+      'skipping_dart2js_compilations_helper.dart');
   var executable = Platform.executable;
   var arguments = [createFileScript, fileUtils.scriptOutputPath.toNativePath()];
   var bootstrapDeps = [
       Uri.parse("file://${fileUtils.testSnapshotFilePath}")];
   var commands = [new runner.CompilationCommand(
       'dart2js',
       fileUtils.testJsFilePath.toNativePath(),
       false,
       bootstrapDeps,
       executable,
@@ skipping 79 matching lines @@
       cleanup();
       throw error;
     }).then((_) {
       cleanup();
     });
   }
   // We need to wait some time to make sure that the files we 'touch' get a
   // bigger timestamp than the old ones
   new Timer(new Duration(seconds: 1), touchFilesAndRunTests);
 }