I think you actually answered your own question.
The difference is that Angular was already a hugely adopted and relatively simple piece of technology (the difference between a JavaScript framework and all the moving parts and pieces of Flutter/Dart is huge)
Since the latter doesn’t have anywhere near the investment outside Google it’s not a fair comparison. The fear that if Google drops it, it will die are legitimate.
> The fear that if Google drops it, it will die are legitimate
I'm not trying to argue against the fact it would die or not. I'm mainly trying to understand why people think Google would abandon it. Abandoning Google Reader and abandoning Flutter or GoLang are _not_ the same in my eyes, and I can't think of any cases where Google has abandoned a technology rather than a consumer facing product in this fashion.
In the .NET block down the street, we deal with this kind of nonsense almost daily :) https://news.ycombinator.com/item?id=41195650
Angular came out of Google and hasn't gone anywhere. Also GoLang.. even GWT was supported well past its prime and is now maintained by the community. What evidence is there that they abandon languages and frameworks?