Is that an indictment of Go, or just an observation that a usable platform is an interconnected set of tools?
I guess my point is that, yes, in each case there's an interconnected set of tools, and in the case of Go it's called "Go" or "Golang". It's a GC language with good kernel bindings but poor interop otherwise, so it's good for low-level stuff that runs directly on top of a kernel, like network servers and CLI tools.
In the Dart case the interconnected set of things is called "Flutter" and it's a cross-platform UI toolkit that happens to use its own language. Exactly the same category as Qt, in fact -- I can't even remember what Qt's special C++ variant is called, if it even has a name. I'm not aware of Qt's C++ extensions being used anywhere else.
You could definitely argue that they're both in a niche, and the UI niche is a nice big one. It feels like Dart could expand to other areas (more easily than Go could be used for a UI) but for whatever reason it hasn't. Similar to server-side Swift.
It's a question of whether the tail is wagging the dog. Flutter is more important than Dart, unless Dart finds a way to expand into another niche. I don't think any one Go framework is bigger than the language itself (even if you were to include the standard library networking utils as a framework).