Flutter isn't a competitor to electron as you can't target web browsers with it.

I tried Flutter over a year ago and found the instant reloading of code nice and has most of modern features but went back to native as the library support is still non-existent.

The real competitor to flutter is kotlin cross-platform from jetbrains. Most android developers are already familiar with kotlin, unlike dart, and also has great tooling.

I look forward to seeing which one wins mind share as they are well designed options.