Fix some minor static type issues in dart:io implementation.

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);
     _data = [];
     _position = 0;
     _file.onError = (e) {
@@ skipping 637 matching lines @@
           "Mixed use of synchronous and asynchronous API");
     }
     var decoder = _StringDecoders.decoder(encoding);
     List<int> bytes = readAsBytesSync();
     decoder.write(bytes);
     return _getDecodedLines(decoder);
   }
 
   String get name() => _name;
 
-  void set onError(void handler(String error)) {
+  void set onError(void handler(Exception error)) {
     _onError = handler;
   }
 
   void _ensureFileService() {
     if (_fileService == null) {
       _fileService = _FileUtils.newServicePort();
     }
   }
 
   bool _isErrorResponse(response) {
@@ skipping 429 matching lines @@
   bool _asyncUsed;
   int _pendingWrites = 0;
 
   SendPort _fileService;
 
   Timer _noPendingWriteTimer;
 
   Function _onNoPendingWrites;
   Function _onError;
 }