Add a flush operations to IOSink

tests/standalone/io/file_write_as_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.
 
 import 'dart:async';
 import 'dart:io';
 
 import "package:async_helper/async_helper.dart";
 import "package:expect/expect.dart";
 
 testWriteAsBytesSync(dir) {
   var f = new File('${dir.path}/bytes_sync.txt');
-  var data = [50,50,50];
+  var data = [50, 50, 50];
   f.writeAsBytesSync(data);
   Expect.listEquals(data, f.readAsBytesSync());
-  f.writeAsBytesSync(data, mode: FileMode.APPEND);
+  f.writeAsBytesSync(data, mode: FileMode.APPEND, flush: true);
   var expected = [50, 50, 50, 50, 50, 50];
   Expect.listEquals(expected, f.readAsBytesSync());
 }
 
 testWriteAsStringSync(dir) {
   var f = new File('${dir.path}/string_sync.txt');
   var data = 'asdf';
   f.writeAsStringSync(data);
   Expect.equals(data, f.readAsStringSync());
-  f.writeAsStringSync(data, mode: FileMode.APPEND);
+  f.writeAsStringSync(data, mode: FileMode.APPEND, flush: true);
   Expect.equals('$data$data', f.readAsStringSync());
 }
 
 Future testWriteAsBytes(dir) {
   var completer = new Completer();
   var f = new File('${dir.path}/bytes.txt');
-  var data = [50,50,50];
+  var data = [50, 50, 50];
   f.writeAsBytes(data).then((file){
     Expect.equals(f, file);
     f.readAsBytes().then((bytes) {
       Expect.listEquals(data, bytes);
-      f.writeAsBytes(data, mode: FileMode.APPEND).then((file) {
+      f.writeAsBytes(data, mode: FileMode.APPEND, flush: true).then((file) {
         Expect.equals(f, file);
         f.readAsBytes().then((bytes) {
           var expected = [50, 50, 50, 50, 50, 50];
           Expect.listEquals(expected, bytes);
           completer.complete(true);
         });
       });
     });
   });
   return completer.future;
 }
 
 Future testWriteAsString(dir) {
   var completer = new Completer();
   var f = new File('${dir.path}/strings.txt');
   var data = 'asdf';
   f.writeAsString(data).then((file){
     Expect.equals(f, file);
     f.readAsString().then((str) {
       Expect.equals(data, str);
-      f.writeAsString(data, mode: FileMode.APPEND).then((file) {
+      f.writeAsString(data, mode: FileMode.APPEND, flush: true).then((file) {
         Expect.equals(f, file);
         f.readAsString().then((str) {
           Expect.equals('$data$data', str);
           completer.complete(true);
         });
       });
     });
   });
   return completer.future;
 }
 
 main() {
   asyncStart();
   var tempDir = Directory.systemTemp.createTempSync('dart_file_write_as');
   testWriteAsBytesSync(tempDir);
   testWriteAsStringSync(tempDir);
   testWriteAsBytes(tempDir).then((_) {
     return testWriteAsString(tempDir);
   }).then((_) {
     tempDir.deleteSync(recursive: true);
     asyncEnd();
   });
 }