long pause opening flutter gallery on slow network

On a Mac laptop with a slowish network connection:
- git clone https://github.com/flutter/flutter.git
- Open examples/flutter_gallery in IDEA

A progress dialog appears that we are opening the project. The progress bar is frozen, for a minute or so. If you are patient, it does eventually complete, but we should show more progress here.

(Behind the scenes it is presumably downloading the Dart SDK.)

Reproduce the pause by cloning flutter/flutter on a Mac with a slow network and opening examples/flutter_gallery in IDEA. Observe the progress dialog while the Dart SDK is presumably downloading; done means the opening flow reports meaningful progress instead of appearing frozen until completion.

Written by the indexing model from the issue text.