Hi @andrew, We don't have any kind of pre-built component library right now.
I agree w/ you and @gabrycir that it can certainly make things faster when you have pre-built pieces of operating system UI you can throw together when designing your app.
We don't have that right now because:
1) It's not a core differentiator of our product, so we haven't prioritized it. We'd definitely love to have it, but we want to make sure we get things like the layout engine working well first.
2) Sharing components is a really big topic, and we're trying to better understand everyone's workflow before we try and tackle it. E.g., How many people just want to start with pre-bulit stuff that doesn't change often (like the iOS status bar)? How many people need to use a component library that does change often? To keep 10 designers on the same project synced up, for example? What happens when a component changes in the library --- does it update all of your local designs? Or do you have to manually "update" the component in your file? What happens when you make a local change to a component and it changes in the library? (a "merge conflict") Is there a UI for that? Do you just have to pick one and lose the changes in the other? Etc.
Lots of tricky problems w/ a component library, depending on the scope. We'll be talking about these issues more once the "basics" settle down.
If you have specific use cases or ideas in the mean time, though, (or great examples from other software of sharing/syncing assets), definitely add 'em to this thread.