Summary:
Removed an array.slice() call in the callImmediatesPass, which seems unnecessary to me, as the variable is immediately re-assigned on the next line.
Also fixed some flow issues, clarified a systrace marker and opensourced the relevant tests.
Changelog: [Internal] [Fixed] Improved JSTimer efficiency
Reviewed By: yungsters
Differential Revision: D20039181
fbshipit-source-id: 9b146980e8fa9f94b2f6153cc67cc7ced58104e5