Using Dart in flutter feels like Google was trying to avoid tossing away a bunch of pre-existing work by finding a purpose for it somewhere. It's Lars' sorta Java, sorta Javascript personal project and it just doesn't have any sort of design to it. Dart's failure in the web space showed that there wasn't any real demand for it - it just wasn't better enough.
I realize they can't use Java, but this is a weird horse to tie themselves to.
They certainly did find a new purpose for Dart outside of it being used in parts of their Ads platform, but
Flutter, when is was the chrome experiment called sky, first started out using Javascript.
The chrome guys decided to move on and look at several language before going with Dart, Swift may have been an option, if it was opensource at the time.
Seems a reason for Dart's failure in the web space was mobile.
Lar gave a talk last year about his and Kasper iot startup https://www.toitware.com/ he mentions why dart didn't make it into chrome, saying "then the whole mobile shift happen and there was no room for extra stuff in chrome"
https://youtu.be/mPna6D21Eqg
Going with a less-popular language without a killer app yet has its advantages. When you're making a new UI platform it's helpful if you can ask the language team to make changes to the language for you.
Once a language is in use in many places, you can't do that anymore.
Apple did this too when they wanted a language that was compatible with Objective C.
And this is how JavaScript got started, after all.
Its a really, really good language - there's links in another post to programming usage ranking blogs, Dart has spiked over the last year, and they note "for languages, frameworks lead adoption"
I realize they can't use Java, but this is a weird horse to tie themselves to.