Convert TargetEventHandlers from class to function

[lencioni]

The code added by Babel to compile this down to ES5 adds about 1.1 KiB
to the built version. We can pretty trivially avoid this for now by
using a function instead of a class. This drops the built filesize from
7.3 KiB to 6.2 KiB, which will be a small win for performance.

I think Babel 7 aims to make some changes that will help us use the
class syntax while avoiding this unnecessary overhead, at which point we
might want to re-evaluate this change. But for now, let's take the easy
bundle size win.