Summary:
In profiling our app, we found that the usage
of `merge` in `Text.js` was showing up as a
hotspot. We've replaced this usage of `merge`
with `mergeFast`.
**Test plan (required)**
This change is used in my team's app.
Adam Comella
Microsoft Corp.
Closes https://github.com/facebook/react-native/pull/9654
Differential Revision: D3801791
fbshipit-source-id: 004652ed6537b557d00541ab2e5fbe64b56fa73b
5d7227a822