> Dart overlaps in goals almost completely with Typescript
As I see it, that's not all that true.
Now, it's true, that it's non-native use competes with TypeScript in domain and shares some implementation method details in that domain (e.g., both are languages that rely on compilation to JS in the web domain), but the goals clearly aren't the same. TypeScript's goals are basically limited to static typing and source compatibility with modern JavaScript (that is, it is an goal to be a typed superset of JS.)
Dart has a wide array of different goals, including being a typed replacement for JS that is not limited in its other goals by being source-compatible with JS.
Dart overlaps in goals almost completely with Typescript which is heavily used multiple teams within Google.
Switching Flutter to Typescript is something I could easily see happening if Google wanted Flutter to be truly popular.