If Flutter and Dart were donated to the community, they would most likely slowly die because no one outside of Google gets enough benefit from them to justify maintaining an entire programming language.
You could suggest using AI to OCR the canvas but even that would be subpar because most sites that use HTML provide multiple screens worth of info but sites that render to a canvas render only what's visible. The rest of the data is internal. You'd not only need the AI to successfully OCR the text, you'd need it to interact with the page get it to render what's not currently displayed.
I think you should ask the opposite question instead, and this goes for any project that is corporate "owned"/sponsored like Flutter: why should we expect the community to maintain it if the company abandons it?
Are there any non-Googlers with commit access to the project? Do non-Googlers comprise a decent percentage of contributions? If not, the bar goes up quite a bit for a fork, or for a "peaceful handoff" of the existing project.