Your consuming code is tied to Foo. In dart, the consuming code only needs tying up to the interface.

> You always need a class

Nope. That's the whole point which I am trying to communicate: you do not need a reference to a class to get the instance of the interface from, as you confirm later on:

> As to the calling code needing to know about that one concrete implementation:

My personal feeling of liberation comes from the fact that the code is simpler and more clear. The non-coupling is nice, but can be achieved with dependency injection too: the point is the transfer of characters into a mental model which is more efficient with the named constructors/factory constructors.