Reapply part of r5325.

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 479 matching lines @@
   }
 
   RandomAccessFile openSync([FileMode mode = FileMode.READ]) {
     if (_asyncUsed) {
       throw new FileIOException(
           "Mixed use of synchronous and asynchronous API");
     }
     if (mode != FileMode.READ &&
         mode != FileMode.WRITE &&
         mode != FileMode.APPEND) {
-      throw new FileIOException("Unknown file mode. Use FileMode.READ, " +
+      throw new FileIOException("Unknown file mode. Use FileMode.READ, "
                                 "FileMode.WRITE or FileMode.APPEND.");
     }
     var id = _FileUtils.checkedOpen(_name, mode._mode);
     assert(id != 0);
     return new _RandomAccessFile(id, _name);
   }
 
   static RandomAccessFile _openStdioSync(int fd) {
     var id = _FileUtils.openStdio(fd);
     if (id == 0) {
@@ skipping 587 matching lines @@
   bool _asyncUsed;
   int _pendingWrites = 0;
 
   SendPort _fileService;
 
   Timer _noPendingWriteTimer;
 
   Function _onNoPendingWrites;
   Function _onError;
 }