Generate SimpleEventPlugin data structures at runtime (#7616)

We used to copy and paste the same big blob many times in order for it to work with keyOf which is no longer a constraint. This pull request takes a list of all the events as string and generate those data structures at runtime.

It reduces the size of React by 1k post gzip and flow is able to extract the structure out of it :)
(cherry picked from commit 1229a23)