legacy-data-mixin doesn't work with wildcards observers

[vdonich]

Observer is being called despite undefined property value.
Change record is valid, but all value-related properties such as changeRecord.base and changeRecord.value are undefined:

{base: undefined, path: "alwaysUndefinedObject", value: undefined}

Here's the jsfiddle with example:
https://jsfiddle.net/gaj0umLs/26/

Originally posted by @vdonich in #5266 (comment)

[bro314]

Hey Kevin, can you give an update on this? This is blocking the Gerrit Polymer 2 migration at the moment. Thanks, Ben