assert.dom('foo', 'bar') should throw an error

[Turbo87]

I managed to write an assertion like assert.dom('foo', 'bar') today, where I actually meant assert.dom('foo').hasText('bar') 😱

This is clearly a bug, and actually one that we would be able to catch. The .dom() function does accept an optional second argument, but if it is not undefined then it should be an Element (or at least have querySelector() and querySelectorAll() methods).

I would propose that in the .dom() we check that, if a rootElement is passed to the function, the rootElement has querySelector() and querySelectorAll() methods and otherwise we throw an error, which would have prevented the issue above.