Preview: Introduce reactNativeComponent

[sahrens]

Summary:

Creating a new native component is a pain - you have to write native
code, manually export the properties you want to use in JS, then copy those
props in JS in validAttributes and make sure you specify differs for
non-scalar props, otherwise changes won't be sent to native.

This diff adds native code to automatically export nativePropTypes, and
consumes those in a new helper requireNativeComponent. Right now the actual
types aren't used in JS, but I changed the logic to default to using
deepDiffer for objects without an explicit differ specified (this should be
much safer in general - we've had a few bugs related to bad diff configuration).
The only disadvantage to always using deepDiffer for Objects is perf, so
requireNativeComponent has an option to provide optimized differ functions,
which is demonstrated in ScrollView.js.

The native export is done similarly to the way we figure out property setters,
but with class methods (prefixed with getViewPropDef_) that simply return
dictionary literals with the type info.

This also sets the stage for potentially doing JS-side type checking, which
would give us better errors with more context and useful stack traces.

Finally, this gives us the ability to warn or throw when requiring unsupported
native views.

cc @ide - this is a preview of an internal diff that's under review.

Test Plan:

SliderIOS still works, sticky headers stick properly (they break if
diffing breaks), other apps/interactions seems fine.

[ide]

This looks great and I think it makes the right tradeoffs. You might want to prefix the codegen'd methods with rct_ which is Apple's suggested convention for private methods, although in practice I don't see name collisions happening.

A longer-term thing to think about is Swift support... since Swift can use the Objective-C runtime API it should be possible to dynamically add the getViewPropDef_* methods but there isn't a way to do it with a macro. Just something to keep in mind, but it feels like there's a path forward to accommodate Swift one day.

[sophiebits]

This happened, I think.