Summary:
`Element` is an abstraction layer that allows describing component hierarchy in a declarative way. Creating `Element`s themself does not create a component tree (aka `ShadowNode` tree) but describes some hierarchical structure that might be used to build an actual component tree (similar to XML Elements).
`Element<>` provides some basic type-safety guarantees: all modifications of element objects require using objects (such as Props or State) of compatible type.
For now, the only useful application of that is building tests.
Changelog: [Internal] Fabric-specific internal change.
Reviewed By: sammy-SC
Differential Revision: D19512392
fbshipit-source-id: eb0711c2a537865fa5454dbede53412a135058cf
6a1438c044