Explore tracking node focus with classname

Let's face it...writing a Mocha test that finds the node with the global focus of a document is a nightmare. PhantomJS intermittently fails and you have to restart TravisCI a few times a week due to unrelated tests that fail.

What if there was a classname...say .slds-focused or something like that which could be queried to find focus, then documentElement wouldn't have to be referenced. When the component controls focus (such as left and right arrows in a pill box), focus is set by the view-model itself on render anyway. When the user interacts with the UI (such as a mouse click), this is a different story however, since the DOM is setting focus itself and not the internal view-model, so this class would only be used by view-model-derived focus which would typically be what tests would test--we don't need to test if the browser works.

No source file or specific test path is named. Start by reviewing the Mocha focus tests and the existing focus handling for view-model-controlled components such as pill boxes, including the PhantomJS and TravisCI failures. Done requires an agreed focus-tracking approach that makes the relevant state queryable without relying on documentElement.