dillon1000/react

import runtime from "React.unstable_ForgetRuntime"

Imports the runtime from `React.unstable_ForgetRuntime` rather than from a 
separate module. The hope is that any code that gets transformed already has a 
dependency on React anyway, so we can avoid adding a new dependency that other 
systems don't know about. 

While here, i also cleaned up the `guardThrows` flag (we still parse it if 
present and warn, rather than throwing, to make it easier to adopt the latest 
version in various places).