Fix bot failure: use platform specific separator

tests/standalone/io/file_write_only_test.dart

Index: tests/standalone/io/file_write_only_test.dart
diff --git a/tests/standalone/io/file_write_only_test.dart b/tests/standalone/io/file_write_only_test.dart
index 6b00ac4b960847bf6e29dd54873e35367979966d..4b58bd05eba96635aa87c2d70ac5ed9626adcc92 100644
--- a/tests/standalone/io/file_write_only_test.dart
+++ b/tests/standalone/io/file_write_only_test.dart
@@ -35,7 +35,7 @@ Future expectThrowsAsync(Future future, String message) {
 
 
 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);
@@ -55,7 +55,7 @@ Future write(Directory dir) async {
 }
 
 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);
@@ -66,7 +66,7 @@ void writeSync(Directory dir) {
 }
 
 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();