The HttpRequest class in Asp.Net 5 (vNext) contains (amongst other things) parsed details about the URL for the request, such as Scheme, Host
It looks like you can't access it directly, but you can build it using the framework:
Microsoft.AspNetCore.Http.Extensions.UriHelper.GetFullUrl(Request)
You can also use the above as an extension method.
This returns a string rather than a Uri, but it should serve the purpose! (This also seems to serve the role of the UriBuilder, too.)
Thanks to @mswietlicki for pointing out that it's just been refactored rather than missing! And also to @C-F to point out the namespace change in my answer!