Fix type assertion error in dart:io File library.

runtime/bin/file_impl.dart

 // Copyright (c) 2012, 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.
 
 class _FileInputStream extends _BaseDataInputStream implements InputStream {
   _FileInputStream(String name) {
-    _file = new File(name);
+    var file = new File(name);
     _data = [];
     _position = 0;
-    _file.onError = _reportError;
-    _file.open(FileMode.READ, (openedFile) {
+    file.onError = _reportError;
+    file.open(FileMode.READ, (openedFile) {
       _readDataFromFile(openedFile);
     });
   }
 
   _FileInputStream.fromStdio(int fd) {
     assert(fd == 0);
-    _file = _File._openStdioSync(fd);
+    var file = _File._openStdioSync(fd);
     _data = [];
     _position = 0;
-    _readDataFromFile(_file);
+    _readDataFromFile(file);
   }
 
   void _readDataFromFile(RandomAccessFile openedFile) {
     openedFile.onError = _reportError;
     openedFile.length((length) {
       var contents = new ByteArray(length);
       if (length != 0) {
         openedFile.readList(contents, 0, length, (read) {
           if (read != length) {
             _reportError(new FileIOException(
@@ skipping 36 matching lines @@
     _position += len;
     _checkScheduleCallbacks();
     return len;
   }
 
   void _close() {
     if (_closed) return;
     _closed = true;
   }
 
-  File _file;
   List<int> _data;
   int _position;
   bool _closed = false;
 }
 
 
 class _FileOutputStream extends _BaseOutputStream implements OutputStream {
   _FileOutputStream(String name, FileMode mode) {
     _pendingOperations = new List<List<int>>();
     var f = new File(name);
@@ skipping 954 matching lines @@
   String _name;
   int _id;
   int _pendingWrites = 0;
 
   SendPort _fileService;
 
   Timer _noPendingWriteTimer;
 
   Function _onNoPendingWrites;
 }