Hi Guix!
As announced on guix-devel, I bootstrapped the dart compiler from
source.

Following patches add dart 2.8.4 (latest release is 2.10, but I wanted
to be sure that this work in progress is fine before continuing, as I
fear that 4 other steps are required).

NOTE: I tagged this as WIP as I've not yet disabled analytics yet (I
disabled it from the first added version, but I was wondering if I need
to disable it in _each_ version or if just the latest one is fine.
Patching all of them will require some time).

(Each build takes ~20min on my server and ~60min on my laptop, and
there's a dozen of them)

Let me know if there are major problems or if I can go on with disabling
analytics!

Thanks, Nicolò