I paste my log here.
$ ./mach robocop
/home/tlin/Projects/gecko/obj-fennec/backend.TestManifestBackend is out of date with respect to /home/tlin/Projects/gecko/testing/mochitest/mach_commands.py
Test configuration changed. Regenerating backend.
Re-install Firefox? (Y/n) n
Error running mach:
The error occurred in the implementation of the invoked mach command.
This should never occur and is likely a bug in the implementation of that
You can invoke |./mach busted| to check if this issue is already on file. If it
isn't, please use |./mach busted file| to report it. If |./mach busted| is
misbehaving, you can also inspect the dependencies of bug 1543241.
If filing a bug, please include the full output of mach, including this error
The details of the failure are as follows:
TypeError: run_robocop_test() takes exactly 3 arguments (110 given)
File "/home/tlin/Projects/gecko/testing/mochitest/mach_commands.py", line 579, in run_robocop
return mochitest.run_robocop_test(self._mach_context, tests, 'robocop', **kwargs)