"pub serve" appears to be running dart2js on every single entrypoint, instead of just the one I navigate to.

This is by design. In general we want to start running transforms as soon as "pub serve" starts up so that we're more likely to have the fully-transformed assets available when the user requests them. Unfortunately when there are many slow transforms that aren't necessary for every web page, this heuristic doesn't work as well.

We've talked a little about adding optimizations in barback to prioritize transformers needed to fulfill a request once that request comes in. I don't think we have a bug for that yet, so I'm going to hijack this one for the purpose.

Removed Area-Pub label.
Added Library-Barback, Area-Pkg labels.
Changed the title to: "barback should prioritize transforms that are needed to produce assets with active requests".

sounds good to me :)

Removed Library-Barback label.
Added Pkg-Barback label.

This issue has been moved to dart-archive/barback#23.