Allow selection.style getter in event handlers

[alexcjohnson]

Fixes #2244

@monfera you're right, d3.svg.brush doesn't work in a way that my idea of disabling the check inside it would work. But here's an alternative: if d3.event is truthy then we know we're in a user-initiated event, and so at least within the contexts of our own test systems we know the plot must be visible because the user is interacting with it!

Seem reasonable?

[monfera]

That's a fantastic idea 💃 ! Event handlers are often used to initiate userland changes to the scenegraph via style or attribute setting, eg. changing the color of something on hover, while d3.event is truthy. So, to clamp down on permissiveness, we could combine this with the cursor check. Probably D3 itself has very limited use of getters and we can add to the whitelist one by one if we encounter another, or we grep all D3 uses from the sources beforehand. But maybe I'm overthinking it, it's a great PR as it is!

[alexcjohnson]

Great, I'll merge it. As the whole initial render of any chart happens in our image tester and test dashboard with d3.event=null, I can't see much that we could be missing by allowing this exception as it is here, and I'd rather not accidentally break any other acceptable uses by trying to be overly cautious.