Summary: Split up InitializeCore into a bunch of modules. The idea here is to make it easier for apps to just get the initialization logic they want and leave behind what they don't; for example, if you don't want the Map/Set polyfills, instead of requiring InitializeCore you can require the modules you want from it.
Reviewed By: yungsters
Differential Revision: D10842564
fbshipit-source-id: 3b12d54fddea8c4ee75886022338c214987a015c
Emily Janzer