# File lib/minitest/unit.rb, line 99
    def assert_instance_of cls, obj, msg = nil
      msg = message(msg) { "Expected #{mu_pp(obj)} to be an instance of #{cls}, not #{obj.class}" }
      flip = (Module === obj) && ! (Module === cls) # HACK for specs
      obj, cls = cls, obj if flip
      assert obj.instance_of?(cls), msg
    end