To share a bit more detail. I'm seeing these times on my local machine when building todomvc:

todomvc with polymer 0.10.0-pre.2

at 0s -- invoke pub-build
at 21s -- pub prints "Building todomvc ..."
at 23s -- first dart2js info message (probably transformers are done?)
at 36s -- build ends.

I'm preparing some changes in my transformer that add even more time. In particular, I'm adding dependencies on the analyzer, and I'm adding transformers to other packages that polymer depends on. This seemed to have a big cost in the bootstrap time. Here are the times with my changes:

at 0s -- start
at 40s -- "Building todomvc ..."
at 46s -- dart2js begins
at 58s -- end

It would be good to share what is going on during the first chunk of time (40s with my changes).

Set owner to @munificent.
Added Started label.

Added Fixed label.

This issue has been moved to dart-lang/pub#917.