Thanks Chad.  It's intentional for certain properties in the API to not be documented
yet.  If something is not documented, it's not a good idea to reference them since
they may change at any point in the future.

Noted. Actually I just saw the note on the object about it not being changed after
creation in the documentation. Must have missed that before.

Other people in the groups are using the properties in examples. Can you obfuscate
the properties so it's clear that they shouldn't be used?

I had hoped it'd be more implicitly understood that non-documented objects,
properties, and methods should not be referenced.  However, you just proved this may
not always be the case.

I'll discuss with the team what we can do here to prevent undocumented features from
being referenced.

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This seems to leave us with no way to retrieve the URL of a MarkerImage? 

I have a function that tries to turn my map into a KML, and the most straightforward
way of doing that is interrogating the markers to get their properties.