Remove StreamSink(replaced by EventSink) and make IOSink extend EventSink.

tests/standalone/io/file_output_stream_test.dart

Index: tests/standalone/io/file_output_stream_test.dart
diff --git a/tests/standalone/io/file_output_stream_test.dart b/tests/standalone/io/file_output_stream_test.dart
index ae3ee5e5881fc32d9ceb43f1f6624bae677a7688..f71c962b667c5c7f3a82715ac7b53387b4426626 100644
--- a/tests/standalone/io/file_output_stream_test.dart
+++ b/tests/standalone/io/file_output_stream_test.dart
@@ -22,7 +22,7 @@ void testOpenOutputStreamSync() {
   file.createSync();
   IOSink x = file.openWrite();
   var data = [65, 66, 67];
-  x.writeBytes(data);
+  x.add(data);
   x.close();
   x.done.then((_) {
     Expect.listEquals(file.readAsBytesSync(), data);