converts Snapshot to an es6 class instead of using prototype assign

[runspired]

Was working on cleaning up some things and just felt this could use a facelift. This isn't for perf, but I'm going to try to isolate it in a profile anyway because I'm personally curious the overall effects of:

• not setting things on the prototype that we're just going to override in the constructor anyway
• (obvs babelified) es6 class definitions vs. prototype definition using the Object.assign pattern

[runspired]

@stefanpenner this is also something that would be a pretty good object to look at the perf of new EmptyObject() vs Object.create(null) on, as it uses it 4x and populates two of them immediately.

[runspired]

cc @bmac for final review / merging

[locks]

fwiw