Avoid using Object.create for sham-less IE8 compatibility

[sophiebits]

Instead of using Object.create, just shuffle the prototypes around so that we don't need to include es5-sham.

[zpao]

Personally, I would prefer to try to write forward thinking JS and backwards support with shims/shams, rather than inlining those workarounds.

The SyntheticMouseEvents change we should definitely take though since that's just pure correctness and not an IE8 issue. Can you pull that out to a separate PR so it can be merged independently?

[sophiebits]

I guess so. :) Figured putting them together would improve the former's chances of getting accepted. ;)

[sophiebits]

Done. I'm personally in favor of not using Object.create because

1. the description of es5-sham on https://github.com/kriskowal/es5-shim doesn't instill confidence in its correctness
2. the alternative code is super simple and is pretty standard for doing inheritance in JS, and
3. (I don't want to include es5-sham if I don't have to)