Change hasSubscribers to hasListener.

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 a6ddbd6742f6f0c0eac079be5439b5057811a53d..eb0a6d826e64f0bd0f66b978543d766f8b8eac8e 100644
--- a/sdk/lib/io/file_impl.dart
+++ b/sdk/lib/io/file_impl.dart
@@ -130,7 +130,7 @@ class _FileStream extends Stream<List<int>> {
   }
 
   void _onSubscriptionStateChange() {
-    if (_controller.hasSubscribers) {
+    if (_controller.hasListener) {
       _start();
     } else {
       _unsubscribed = true;