Add a flush operations to IOSink

tests/standalone/io/file_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 "package:expect/expect.dart";
 import "package:path/path.dart";
 import 'dart:async';
 import 'dart:collection';
@@ skipping 86 matching lines @@
         buffer.addAll(d);
       },
       onDone: () {
         Expect.equals(42, buffer.length);
         // Write the contents of the file just read into another file.
         String outFilename =
             tempDirectory.path + "/out_read_write_stream";
         var file2 = new File(outFilename);
         var output = file2.openWrite();
         output.add(buffer);
-        output.close();
+        output.flush().then((_) => output.close());
         output.done.then((_) {
           // Now read the contents of the file just written.
           List<int> buffer2 = new List<int>();
           new File(outFilename).openRead().listen(
               (d) {
                 buffer2.addAll(d);
               },
               onDone: () {
                 Expect.equals(42, buffer2.length);
                 // Now compare the two buffers to check if they are
@@ skipping 18 matching lines @@
     List<int> buffer = new List<int>(100000);
     for (var i = 0; i < buffer.length; ++i) {
       buffer[i] = i % 256;
     }
     String filename =
         tempDirectory.path + "/out_read_write_stream_large_file";
     File file = new File(filename);
     IOSink output = file.openWrite();
     output.add(buffer);
     output.add(buffer);
-    output.close();
+    output.flush().then((_) => output.close());
     output.done.then((_) {
       Stream input = file.openRead();
       int position = 0;
       final int expectedLength = 200000;
       // Start an independent asynchronous check on the length.
       asyncTestStarted();
       file.length().then((len) {
         Expect.equals(expectedLength, len);
         asyncTestDone('testReadWriteStreamLargeFile: length check');
       });
@@ skipping 1173 matching lines @@
       testWriteStringUtf8Sync();
       testRename();
       testRenameSync();
     });
   }
 }
 
 main() {
   FileTest.testMain();
 }