Is [1] the element you are targeting? If so, mightn't it be better to use the id `messages-back-button` rather than an xpath? Also, our coding standard for locator tuples is to use the `By` in the first element, as in the locators above the one you added.

[1] https://github.com/mozilla-b2g/gaia/blob/master/apps/sms/index.html#L95