As indicated in the docs provided by Günter, create dart_test.yaml in the root of the package:
#dart_test.yaml #run 2 test suites at the same time (I guess, that in 2 different cores) concurrency: 2
Now run
pub run test test / server.dart test / client.dart -pvm, content-shell
If this takes a long time (usually when opening a browser), you can add to the same configuration file:
timeout: none
You can also save part of the -pvm command, the content-shell command, by running the configuration file:
platforms: - vm - content-shell
If this does not work, you can save the time it took me to figure out what happened by doing:
restore folder cache
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