Tests take too long to run #2295
Description
Currently on my MacBook, pub run test takes around 300 seconds. This is far too long. It makes local testing slow, as well as Travis.
I suspect most of the waste comes from repeatedly resolving the Dart SDK, for each package in testing/, and for each test suite that documents each package in testing/. The time can be reduced by not resolving so much, or by converting any end-to-end test, which reads real packages from testing/, into a unit test, which uses a mock SDK or mock package graph, etc.
It's hard to pick a good acceptance criteria. If work is done to reduce the test duration to below 120 seconds, I think we'd be in a functional state.