Assume your current directory is the shared_counter_test folder. To run mdtest for this shared-counter example, first plugin at least two Android/iOS devices and just run

mdtest run --brief \
           --format none \
           --coverage \
           --spec shared_counter.spec \
           shared_counter_test_1.dart shared_counter_test_2.dart

The above command tells mdtest to run in run mode, only spit out test result and print test output using the dart test default format, collect coverage information, use the test spec located at shared_counter.spec and run the test scripts shared_counter_test_1.dart and shared_counter_test_2.dart.

The above command will produce the coverage information in both plus/coverage/cov_run_01.lcov and minus/coverage/cov_run_01.lcov, you can generate the code coverage report by running (assuming your current directory is either plus or minus folder)

mdtest generate --report-type coverage \
                --load-report-data coverage/cov_run_01.lcov \
                --lib lib \
                --output coverage_report

and mdtest will invoke lcov to generate a coverage report under coverage_report folder.