Make File:openRead always have one outstanding read, when invoking callbacks.

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 aa2e495645abb0a6464a060656daad4b7adf2f42..05c33f605b68581652762c514dd82c05b6dd9c90 100644
--- a/sdk/lib/io/file_impl.dart
+++ b/sdk/lib/io/file_impl.dart
@@ -95,20 +95,23 @@ class _FileStream extends Stream<List<int>> {
     }
     _openedFile.read(readBytes)
       .then((block) {
+        _readInProgress = false;
         if (_unsubscribed) {
           _closeFile();
           return;
         }
-        _readInProgress = false;
         _position += block.length;
         if (block.length < readBytes ||
             (_end != null && _position == _end)) {
           _atEnd = true;
         }
-        _controller.add(block);
-        if (!_controller.isPaused) {
+        if (!_atEnd && !_controller.isPaused) {
           _readBlock();
         }
+        _controller.add(block);
+        if (_atEnd) {
+          _closeFile();
+        }
       })
       .catchError((e, s) {
         if (!_unsubscribed) {