Summary:
This shows progress for the download of the JS bundle (different from the packager transform progress that we show already). This is useful especially when loading the JS bundle from a remote source or when developing on device (on simulator + localhost it pretty much just downloads instantly). This will be nice for the expo client since all bundles are loaded over the network and can take several seconds to load.

This depends on facebook/metro#28 to work but won't crash or anything without it, it just won't show the progress percentage.

![img_05070155d2cc-1](https://user-images.githubusercontent.com/2677334/28293828-2c08d974-6b24-11e7-9334-e106ef3326d9.jpeg)

**Test plan**
Tested that bundle download progress is shown properly in RNTester on both localhost + simulator and on real device with network conditionner to simulate a slow loading bundle.

Tested that it doesn't cause issues if the packager doesn't send the Content-Length header.
Closes #15066

Differential Revision: D5449073

Pulled By: shergin

fbshipit-source-id: 43a8fb559393bbdc04f77916500e21898695bac5

Summary:
If the z-index was updated after the initial mount, changes would not be reflected because we did not recalculate the z-index mapped child views and redraw the view. This adds code to do that and call it whenever we update z-index.

**Test plan**
Tested by reproducing the bug with 2 overlapping views that change z-index every second. Made sure it now works properly and z-index changes are reflected.
Closes #15203

Differential Revision: D5564832

Pulled By: achen1

fbshipit-source-id: 5b6c20147211ce0b7e8954d60f8614eafe128fb4