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

sdk/lib/io/file_impl.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart.io;
 
 // Read the file in blocks of size 64k.
 const int _BLOCK_SIZE = 64 * 1024;
 
 
@@ skipping 77 matching lines @@
         if (!_unsubscribed) {
           _controller.addError(new RangeError("Bad end position: $_end"));
           _closeFile();
           _unsubscribed = true;
         }
         return;
       }
     }
     _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;
         }
+        if (!_atEnd && !_controller.isPaused) {
+          _readBlock();
+        }
         _controller.add(block);
-        if (!_controller.isPaused) {
-          _readBlock();
+        if (_atEnd) {
+          _closeFile();
         }
       })
       .catchError((e, s) {
         if (!_unsubscribed) {
           _controller.addError(e, s);
           _closeFile();
           _unsubscribed = true;
         }
       });
   }
@@ skipping 791 matching lines @@
   void _checkAvailable() {
     if (_asyncDispatched) {
       throw new FileSystemException("An async operation is currently pending",
                                     path);
     }
     if (closed) {
       throw new FileSystemException("File closed", path);
     }
   }
 }