All test shards defined in test.dart running on Windows are failing, seemingly due to incorrect reporting of process exit codes.

Examples:

• https://logs.chromium.org/logs/flutter/buildbucket/cr-buildbucket.appspot.com/8842467660402838256/+/u/run_test.dart_for_tool_integration_tests_shard_and_subshard_4_5/stdout
• https://logs.chromium.org/logs/flutter/buildbucket/cr-buildbucket.appspot.com/8842436829389127104/+/u/run_test.dart_for_build_tests_shard_and_subshard_1_3/stdout
• https://logs.chromium.org/logs/flutter/buildbucket/cr-buildbucket.appspot.com/8842428023780813616/+/u/run_test.dart_for_framework_tests_shard_and_subshard_widgets/stdout

And yet LUCI dashboard is all green:

The error in the logs seems to indicate that bin\flutter.bat fails to report a non-zero exit code. The reason why LUCI fails to fail might be because bin\dart.bat has a similar bug.