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

BUG=11685
R=gram@google.com

Review URL: https://codereview.chromium.org//18529009

git-svn-id: http://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/webdriver@25307 260f80e4-7a28-3924-810f-c04153c831b5
Merge branch 'master' of https://github.com/dart-lang/webdriver