The running e.g. flutter test --coverage each test file (i.e. after each vm run of a test file) will be passed through package:coverage which - because of the option to be able to check for ignored lines - has to read all files in the source report (in the package(s) we're getting coverage for). With many test files (and depending on the dependency graph, but still) it will then keep reading the same files over and over.

In dart-archive/coverage#391 I added the ability to cache these reads and this PR utilizes this in flutter.
This makes each file only be read at most once.

Trying it out in packages/flutter before this PR I got this:

$ time flutter test --coverage

[...]

13:45 +10875 ~66 -1: Some tests failed.                                                                                                                                                        

real    13m48.726s
user    78m56.959s
sys     10m21.363s

With this PR I instead get this:

$ time flutter test --coverage

[...]

10:13 +10875 ~66 -1: Some tests failed.                                                                                                                                                        

real    10m17.241s
user    75m49.775s
sys     10m7.498s

I.e. from ~828 seconds to ~617 seconds, or a ~25% reduction in time spent (testing without coverage takes ~236 seconds so the overhead of --coverage in this case goes from ~592 seconds to ~381 seconds, or a ~35% reduction).