Part of the possible completions involves evaluating the part
of the passed in text that looks like an object, so that we
can query it for properties.
Using a Function or eval() for that means that we can only
complete text that does not use `await`. To get over that
limitation, evaluate the text in an AsyncFunction instead.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2842>
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