Phaser is tied to JS, that is first class in browsers and will be for a while. WASM being de facto is far from a certainty, and a long way off in the best case for it (it may see more initial penetration in tooling than applications).
Tying your project to a tech like Flutter (or even Dart) is ok, but can also potentially be a killer for your project.
TS isn't immune from this either (see CoffeeScript) but its far less invasive. If TS died tomorrow (not that it would, barring some crazy licensing thing) deleting the TS bits is trivial and can be handled by a codemod and you haven't tanked your project. Not so for writing in a completely separate language.
Not that I don't think a Dart/Flutter version is a bad idea per se, but, for me, it would be a port (or a totally separate project that follows the same/or similar conventions) with Phaser remaining as a JS (TS) project to ensure longevity of the project and remove barriers to entry.