Fix bot failure: use platform specific separator

tests/standalone/io/file_write_only_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.
 //
 // Dart test program for testing file I/O.
 
 import 'dart:async';
 import 'dart:io';
 
 import "package:async_helper/async_helper.dart";
@@ skipping 17 matching lines @@
   }
 }
 
 Future expectThrowsAsync(Future future, String message) {
   return future.then((r) => Expect.fail(message))
                .catchError((e) {});
 }
 
 
 Future write(Directory dir) async {
-  var f = new File(dir.path + '/write');
+  var f = new File("${dir.path}${Platform.pathSeparator}write");
   var raf = await f.open(mode: WRITE_ONLY);
   await raf.writeString('Hello');
   await raf.setPosition(0);
   await raf.writeString('Hello');
   await raf.setPosition(0);
   await expectThrowsAsync(
       raf.readByte(), 'Read from write only file succeeded');
   await raf.close();
   raf = await f.open(mode: WRITE_ONLY_APPEND);
   await raf.writeString('Hello');
   await expectThrowsAsync(
       raf.readByte(), 'Read from write only file succeeded');
   await raf.setPosition(0);
   await raf.writeString('Hello');
   await raf.close();
   Expect.equals(f.lengthSync(), 10);
 }
 
 void writeSync(Directory dir) {
-  var f = new File(dir.path + '/write_sync');
+  var f = new File("${dir.path}${Platform.pathSeparator}write_sync");
   var raf = f.openSync(mode: WRITE_ONLY);
   raf.writeStringSync('Hello');
   raf.setPositionSync(0);
   raf.writeStringSync('Hello');
   raf.setPositionSync(0);
   Expect.throws(() => raf.readByteSync());
   raf.close();
 }
 
 Future openWrite(Directory dir) async {
-  var f = new File(dir.path + '/open_write');
+  var f = new File("${dir.path}${Platform.pathSeparator}open_write");
   var sink = f.openWrite(mode: WRITE_ONLY);
   sink.write('Hello');
   await sink.close();
   sink = await f.openWrite(mode: WRITE_ONLY_APPEND);
   sink.write('Hello');
   await sink.close();
   Expect.equals(f.lengthSync(), 10);
 }
 
 main() async {
   asyncStart();
   await withTempDir('file_write_only_test_1', write);
   withTempDirSync('file_write_only_test_2', writeSync);
   await withTempDir('file_write_only_test_3', openWrite);
   asyncEnd();
 }