Add a flush operations to IOSink

sdk/lib/io/file_impl.dart

Index: sdk/lib/io/file_impl.dart
diff --git a/sdk/lib/io/file_impl.dart b/sdk/lib/io/file_impl.dart
index 71dca70cef7204961ecce61f26c8437a4542c4f9..e6acc5abd88c76f48fcfeb994693ad332f13fa0e 100644
--- a/sdk/lib/io/file_impl.dart
+++ b/sdk/lib/io/file_impl.dart
@@ -206,6 +206,12 @@ class _FileStreamConsumer extends StreamConsumer<List<int>> {
   Future<File> close() {
     return _openFuture.then((openedFile) => openedFile.close());
   }
+
+  Future flush() {
+    return _openFuture
+        .then((openedFile) => openedFile.flush())
+        .then((randomAccessfile) => new Future.value(null));
+  }
 }
 
 
@@ -482,26 +488,35 @@ class _File extends FileSystemEntity implements File {
   }
 
   Future<File> writeAsBytes(List<int> bytes,
-                            {FileMode mode: FileMode.WRITE}) {
+                            {FileMode mode: FileMode.WRITE,
+                             bool flush: false}) {
     try {
       IOSink sink = openWrite(mode: mode);
       sink.add(bytes);
-      sink.close();
+      if (flush) {
+        sink.flush().then((_) => sink.close());
+      } else {
+        sink.close();
+      }
       return sink.done.then((_) => this);
     } catch (e) {
       return new Future.error(e);
     }
   }
 
-  void writeAsBytesSync(List<int> bytes, {FileMode mode: FileMode.WRITE}) {
+  void writeAsBytesSync(List<int> bytes,
+                        {FileMode mode: FileMode.WRITE,
+                         bool flush: false}) {
     RandomAccessFile opened = openSync(mode: mode);
     opened.writeFromSync(bytes, 0, bytes.length);
+    if (flush) opened.flushSync();
     opened.closeSync();
   }
 
   Future<File> writeAsString(String contents,
                              {FileMode mode: FileMode.WRITE,
-                              Encoding encoding: UTF8}) {
+                              Encoding encoding: UTF8,
+                              bool flush: false}) {
     try {
       return writeAsBytes(encoding.encode(contents), mode: mode);
     } catch (e) {
@@ -511,7 +526,8 @@ class _File extends FileSystemEntity implements File {
 
   void writeAsStringSync(String contents,
                          {FileMode mode: FileMode.WRITE,
-                          Encoding encoding: UTF8}) {
+                          Encoding encoding: UTF8,
+                          bool flush: false}) {
     writeAsBytesSync(encoding.encode(contents), mode: mode);
   }