Minor improvement to File diagnostics

dart/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 475 matching lines @@
         return new FileIOException("File closed");
       default:
         return new Exception("Unknown error");
     }
   }
 }
 
 // Class for encapsulating the native implementation of files.
 class _File extends _FileBase implements File {
   // Constructor for file.
-  _File(String this._name);
+  _File(String this._name) {
+    if (_name is! String) {
+      throw new ArgumentError('${NoSuchMethodError.safeToString(_name)} '
+                              'is not a String');
+    }
+  }
 
   // Constructor from Path for file.
   _File.fromPath(Path path) : this(path.toNativePath());
 
   Future<bool> exists() {
     _ensureFileService();
     List request = new List(2);
     request[0] = _FileUtils.EXISTS_REQUEST;
     request[1] = _name;
     return _fileService.call(request).transform((response) {
@@ skipping 658 matching lines @@
           new FileIOException("File closed '$_name'"));
     });
     return completer.future;
   }
 
   final String _name;
   int _id;
 
   SendPort _fileService;
 }