Web browser app....
> The average user buys a Chromebook:
And returns it back after discovering it cannot install his/her favorute software and requires a permanent internet connection to make anything useful with the device.
> The average user buys a (hypothetical) Dartbook: Looks for some Dart app. Gets frustrated by the lack of choice. Returns the device.
The return rate of those devices running Android is quite big apparently.
> But what use would be a Dartbook to you?
The same as my tablet and mobile phone running Android.