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

pkg/webdriver/lib/webdriver.dart

Index: pkg/webdriver/lib/webdriver.dart
diff --git a/pkg/webdriver/lib/webdriver.dart b/pkg/webdriver/lib/webdriver.dart
index 83ea292c98932c464752205944df305f09f4d70a..25d4b72b7527fd313ee1f50594f48638aac8a78e 100644
--- a/pkg/webdriver/lib/webdriver.dart
+++ b/pkg/webdriver/lib/webdriver.dart
@@ -1,4 +1,4 @@
-// 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.
 
@@ -1168,8 +1168,19 @@ class WebDriverSession extends WebDriverBase {
    * 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).