Found by @mraleph while looking at flutter_tester related things with me.
I took some suggestions from him and added a test. This should eventually help speed up running large multi-file test suites.
The test is a little bit wonky because we don't have great inspection points (e.g. something to mock or examine from a test). It should break if we change the way we load kernel assets in the test harness, and is verifying that we only ask for the test asset once where we used to ask for it twice.