Why is that Flutter does not have a web target? Everything is nice and fast about it, it's a shame that after building a cross mobile apps, you'll app to start the web app from scratch using another platform.
I know for a fast that the core team is open to the idea of creating desktop backends but does not have the resources to do it. They'll gladly accept any contributions towards that.
https://github.com/netdur?tab=repositories
I say porting Flutter to web using canvas (both have similar APIs) is very bad idea.
this repo closes the gap in sane way https://github.com/yjbanov/butterfly
Of note is that Appcelerator did target all three simultaneously (as well as blackberry and windows phone), but eventually dropped web support.