Add a flush operations to IOSink

tests/standalone/io/file_test.dart

Index: tests/standalone/io/file_test.dart
diff --git a/tests/standalone/io/file_test.dart b/tests/standalone/io/file_test.dart
index 6a16efcfff93b78ca48d132e886b9ffe3a536d6d..08a071137edba07c33079fc369465ab82d8fbdff 100644
--- a/tests/standalone/io/file_test.dart
+++ b/tests/standalone/io/file_test.dart
@@ -104,7 +104,7 @@ class FileTest {
         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>();
@@ -143,7 +143,7 @@ class FileTest {
     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;