The world of paid Rust language contributors is so small that Google could easily get all the advantages of "built in-house" for Rust if they spent a relatively tiny amount of money.
Being pretty conservative overall but betting the house on Dart for the UI seems like a strange combination of decisions to me.
They don't seem to be betting on Dart by itself as much as they are betting on Flutter, which is already reasonably successful and relies on familiar reactive component concepts popular on the web and well tested in Google's own web frameworks (Polymer, Angular).
I agree with you, the point I was trying to convey is that Google isn't betting on Dart as a pioneering, untested technology as Rust is.
Flutter demonstrates Dart is a good choice; it gives you a successful declarative UI framework that effectively builds on Dart as a fairly straightforward upgrade of the most tried and true UI scripting language ever made: Javascript.
To answer the GP comment, betting the house on Dart for the UI doesn't seem like a strange or risky decision in that light.
That you like Dart and that it's a good fit for UI development doesn't make it less risky.
It still seems incongruous that widespread usage is portrayed as an important criterion for Fuschia PLs, but they bet big on Flutter which forces them to adopt Dart, a language which has very little uptake outside Flutter.
I'm not sure how you got "I like Dart" from my two comments. I'm clearly saying that Dart & Flutter are based on very popular and well tested concepts/structures, therefore it is not very risky. Dart & Flutter by themselves may not be very widespread, but it's very familiar and easy to adopt to anyone who has done declarative UI web development in Javascript or Javascript-like language, which are widespread.
Rust, on the other hand, is treading new ground with the unusual core concept of a borrow checker.
The languages are as different as Python and C++, I honestly don't really see a Rust vs Dart story. Note that Rust is approved for use within much of the source tree, but outside it is only not supported. Johnny end-developer, whoever that is, could still use Rust if he insisted, coding against C bindings.
If Johnny doesn't like JVM languages, or C++, all they get is a bare bones C API, which requires JNI even for opening files, asking for permissions and so forth.
I don't know what "conspiracy" you might be referring to. There's a natural tendency for organizations to gravitate toward tools, such as programming languages, that originated there. That's just human nature, though, not anything nefarious.
Being pretty conservative overall but betting the house on Dart for the UI seems like a strange combination of decisions to me.