The first "problem" was Javscript.
At one presentation about this I recall that Lars Bak (IIRC) took quite a beating about DArt, just in terms of the questions asked, to the point that someone had to point out just how much he'd accomplished (ie V8). This was, in essence, an appeal to authority.
I recall the Ads PA basically stepping in to save the project. I can only speculate why but my best guess is the AdWords TL woke up one day and decided to rewrite AdWords in Dart (as he allegedly had with GWT some years before). But all that's pure speculation.
The second "problem" that Dart solved was using the same code on client and server, something Google is inexplicably obsessed with (eg GWT). I don't know what caveats came with Dart but GWT had plenty (eg for many years it wasn't possible to use the same protobuf classes on client and server).
I believe the intent with Dart was, at one point, to run natively within Chrome (the article mentions Dartium). This worked out really well for vbscript. No one wants to write different languages for different browsers. Again, I speculate here, but my strong suspicion is the Chrome team killed the idea of shipping a Dart VM in Chrome and that was the end of that.
Maybe it's just me but TypeScript just seemed far more sane.
So... Flutter. I don't know why a UI library needs to be language-specific but it strikes me as a bad idea. Now, the counterpoint is that Flutter is an SDK And more akin to, say, React Native (albeit with significant architectural differences) but I don't think it started that way.
:UI frameworks, libraries and SDKs just don't seem to be in Google's wheelhouse (as opposed to, say, infra, which very much does). I can only speculate as to why but my best guess is Google engineers (disclaimer: Xoogler) can't see passed something as an engineering problem, so much so that they take on philosophies no one asked for or needed (eg same code on client and server), which come with a huge cost and no tangible benefits.
[1]: https://insights.dice.com/2019/03/27/fall-rise-dart-google-j...