Fix type annotation in _FileBase._exceptionFromResponse

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)
       : _data = const [],
         _position = 0,
         _filePosition = 0 {
     var file = new File(name);
@@ skipping 311 matching lines @@
 const int _READ_LIST_REQUEST = 16;
 const int _WRITE_LIST_REQUEST = 17;
 const int _WRITE_STRING_REQUEST = 18;
 
 // Base class for _File and _RandomAccessFile with shared functions.
 class _FileBase {
   bool _isErrorResponse(response) {
     return response is List && response[0] != _SUCCESS_RESPONSE;
   }
 
-  Exception _exceptionFromResponse(response, String message) {
+  _exceptionFromResponse(response, String message) {
     assert(_isErrorResponse(response));
     switch (response[_ERROR_RESPONSE_ERROR_TYPE]) {
       case _ILLEGAL_ARGUMENT_RESPONSE:
         return new ArgumentError();
       case _OSERROR_RESPONSE:
         var err = new OSError(response[_OSERROR_RESPONSE_MESSAGE],
                               response[_OSERROR_RESPONSE_ERROR_CODE]);
         return new FileIOException(message, err);
       case _FILE_CLOSED_RESPONSE:
         return new FileIOException("File closed");
@@ skipping 743 matching lines @@
           new FileIOException("File closed '$_name'"));
     });
     return completer.future;
   }
 
   final String _name;
   int _id;
 
   SendPort _fileService;
 }