Is this actually what we want, given that Roboto isn't on Windows or Linux by default?

Well, I thought about that, but it's the default font for Material, and we're doing Material on these platforms. I think it might look wrong to, say, use "Arial" on Windows and, "San Francisco" on macOS. We can try it, though, if that makes it a smaller download. Don't we package all the fonts in use?

it's the default font for Material

Is it though?

"Roboto is the default system font for Android. For platforms outside of Android and web products, use a system typeface that is preferred on that platform. For example, iOS applications should use Apple’s San Francisco font."
https://material.io/design/typography/understanding-typography.html#system-fonts

Don't we package all the fonts in use?

It's possible to package fonts in an application, but it's not done by default that I'm aware of. Doing it yourself via pubspec doesn't have a per-platform specification that I'm aware of, so I don't think it's possible to have an app that packages Roboto for Windows and Linux without bloating everything else (which would be really bad for iOS and Android), although maybe I'm wrong about that.

Also, if we wanted to auto-package it, then we would need to add tool support to auto-download it, since we can't package something that's not on the machine to start with.

Ahh, I was mistaken about that then. I'll switch it to default to "Segoe UI" on Windows. On Linux, it's not so clear: Ubuntu's default font is "Ubuntu", so I'm guessing that it's not available on Embedded Gentoo. :-)

We should also probably find out if you can specify fonts per platform in a pubspec: someone's definitely going to want that.

We should talk to folks about how to handle Linux. We may need a way to specify a list of a few options depending on what's present, or we may need to pick one that we think is most likely and then rely on font fallback (which is currently awful on Linux per recent discussions, but needs to be fixed anyway).

OK, I added whiteRedmond and blackRedmond typography sets based on "Segoe UI". I left the linux as "Roboto", for lack of a better default for now.

OK, I made linux use Roboto, but added fallbacks of DejaVu Sans, Liberation Sans, and finally Arial.

I'm still not sure about Roboto as the first choice, but we can revisit that later; this is clearly better than nothing. Speaking of revisiting though, will we consider changing this list later to be a breaking change?

That's a tough one. One one hand, rendering run on the same machine might change, on the other, the presence of a particular font is purely a stochastic sample.

I'd lean towards "yes", simply because it might change goldens for some testers.