While running an investigation for shadow root support in Marionette I noticed that the Marionette client fails to serialize ShadowRoot objects in _to_json() with:

ERROR testing/marionette/harness/marionette_harness/tests/unit/test_execute_script.py TestExecuteContent.test_argument_shadow_root - UnboundLocalError: local variable 'wrapped' referenced before assignment
Traceback (most recent call last):
  File "/Users/henrik/code/gecko/testing/marionette/harness/marionette_harness/marionette_test/testcases.py", line 202, in run
    testMethod()
  File "/Users/henrik/code/gecko/testing/marionette/harness/marionette_harness/tests/unit/test_execute_script.py", line 176, in test_argument_shadow_root
    nodeType = self.marionette.execute_script(
  File "/Users/henrik/code/gecko/testing/marionette/client/marionette_driver/marionette.py", line 1727, in execute_script
    args = self._to_json(script_args)
  File "/Users/henrik/code/gecko/testing/marionette/client/marionette_driver/marionette.py", line 1615, in _to_json
    wrapped.append(self._to_json(arg))
  File "/Users/henrik/code/gecko/testing/marionette/client/marionette_driver/marionette.py", line 1632, in _to_json
    return wrapped

The problem here is that we do not support ShadowRoot objects in _to_json() yet.

Due to an issue on Lando with the meanwhile turned off linter for isort the reviewed patch cannot be landed. Sheriffs, could you please land manually on autoland? Thanks a lot.

https://hg.mozilla.org/mozilla-central/rev/23be2debe840