Summary:
When you call `-[RCTConvert UIFont:withFamily:...]` with a non-nil font object, we'll try to use the existing font object for system information. On iOS9+ which uses the San Francisco font, `[UIFont fontNamesForFamilyName:]` doesn't return anything useful, so we need to make sure that we detect this as a system font and use the appropriate methods. This issues is made worse by the fact that RCTTextView and friends recreate the font property for every attribute that is set (pretty horrible perf-wise).
This fixes https://github.com/facebook/react-native/issues/2140
Reviewed By: sahrens
Differential Revision: D3662751
fbshipit-source-id: c528e8945ed361a922c03f861d3c0b584658573b
e30327cb13