Add test for buffered file input stream on dart:io File class.

runtime/bin/stream_util.dart

Index: runtime/bin/stream_util.dart
diff --git a/runtime/bin/stream_util.dart b/runtime/bin/stream_util.dart
index 57c68d312afdb56fd6c8bd8b9f87f8688a079d16..f8856f7af703064a5109b6ab046aaeb418ed69e7 100644
--- a/runtime/bin/stream_util.dart
+++ b/runtime/bin/stream_util.dart
@@ -23,7 +23,7 @@ class _BaseDataInputStream {
   }
 
   int readInto(List<int> buffer, [int offset = 0, int len]) {
-    if (_closeCallbackCalled) return null;
+    if (_closeCallbackCalled) return 0;
     if (len === null) len = buffer.length;
     if (offset < 0) throw new StreamException("Illegal offset $offset");
     if (len < 0) throw new StreamException("Illegal length $len");