Summary:
When code blocks the UI thread for too long, it's a bad sign because this can prevent the app from remaining responsive. This change helps detect such responsiveness issues by warning when a React method executes on the UI thread longer than some threshold.
**Test Plan**
Changed AppState's getCurrentAppState method to sleep for 500 ms and verified that a warning was emitted:
```
2017-08-17 19:45:29.479 [warn][tid:main][RCTModuleMethod.mm:527] mainThreadWatchdog: invocation of [RCTAppState getCurrentAppState:error:] took 501ms
```
Adam Comella
Microsoft Corp.
Closes https://github.com/facebook/react-native/pull/15542
Differential Revision: D5724764
Pulled By: shergin
fbshipit-source-id: f1dc4bf17d3657c397720a47fabc7f32bf81b7ac
68461492bc