Fix moveTo to not include null optional parameters in JSON message.

pkg/webdriver/lib/webdriver.dart

-// Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
+// Copyright (c) 2013, 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.
 
 library webdriver;
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:json' as json;
 
 part 'src/base64decoder.dart';
@@ skipping 1149 matching lines @@
    */
   Future dismissAlert() => _post('dismiss_alert');
 
   /**
    * Move the mouse by an offset of the specificed element. If no element is
    * specified, the move is relative to the current mouse cursor. If an
    * element is provided but no offset, the mouse will be moved to the center
    * of the element. If the element is not visible, it will be scrolled
    * into view.
    */
-  Future moveTo(String id, int x, int y) =>
-      _post('moveto', { 'element': id, 'xoffset': x, 'yoffset' : y});
+  Future moveTo(String id, int x, int y) {
+    Map json = {};
+    if (id != null) {
+      json['element'] = id;
+    }
+    if (x != null) {
+      json['xoffset'] = x;
+    }
+    if (y != null) {
+      json['yoffset'] = y;
+    }
+    return _post('moveto', json);
+  }
 
   /**
    * Click a mouse button (at the coordinates set by the last [moveTo] command).
    * Note that calling this command after calling [buttonDown] and before
    * calling [buttonUp] (or any out-of-order interactions sequence) will yield
    * undefined behaviour).
    *
    * [button] should be 0 for left, 1 for middle, or 2 for right.
    */
   Future clickMouse([button = 0]) => _post('click', { 'button' : button });
@@ skipping 276 matching lines @@
    *
    * 'timestamp' (int) - The timestamp of the entry.
    * 'level' (String) - The log level of the entry, for example, "INFO".
    * 'message' (String) - The log message.
    *
    * This works with Firefox but Chrome returns a 500 response due to a 
    * bad cast.
    */
   Future<List<Map>> getLogs(String type) => _post('log', { 'type': type });
 }