I added a README at the end of https://github.com/flutter/flutter/pull/131901 and did not realize that it was being run as a test, [leading to test failures (of course)](https://logs.chromium.org/logs/flutter/buildbucket/cr-buildbucket/8761789207346423409/+/u/run_test.dart_for_android_preview_tool_integration_tests_shard_and_subshard_None/test_stdout). This makes it so we only run dart files as dart tests.