Just adding a comment because the latest doesn't see to acknowledge that this ability now exists.

First:
Lines and shape need to be converted to path.
To do this use Ctrl+8 (or Command+8 for Apple) or you can simply double-click the line or shape.

Second:
Once the object becomes a path, you will see "Dash" and "Gap" added to the "Size" properties inside the Border section.

Now, go and play.

Still waiting....July 2020.
Why I am on Adobe Xd again?

dashed and dotted lines plus arrow on each end.

Yes, you can do it, in the border section you find "dash" and "gap" and you can play with those values and have different dashed styles

@grinvaldsjanis - you cant' because XD can't convert lines to paths (outline strokes) :-(

and how to convert it to path?

Yep!! Finally available! Took way to long, but it's live.

There's a weird prioritisation of features going on. Basics such as this should be built and shipped before deeper functionality features like "Comment on design specs" or "Private Invites (Beta)". Any update on this feature?

....so?

Please add this feature :-(, wait so long

Feature started since March 22, 2018 It's now July 7, 2018.

need this feature :-(

I can't believe that this guys, being ADOBE, missed the dashed / dotted lines. I can't just believe it. Can someone explain me why?.

That's great!

Any update in this feature

It would be great if you add the dotted/dashed line option in the border settings.

Would love this feature right now. Please add the ability to animate them as well ;)

we are waiting

YEAH! Finally!

we need this.
Anything I can create in CSS should be supported in XD.
Make it happen

This is one of the utmost basic function that ANY vector design application/software must have from day one of the life cycle, not something that users have to request via feature suggestion page!!!