ExtraDataMixin has a series of checks to determine if it should

set this.extraData. When calling set(), it makes a determination

based on the attributes passed in and the current object. This doesn't

always do the right thing, though.
If attributes are passed in and this.extraData is not set, then we

fail when trying to call a method on it.
Along with that, the method used to check this.extraData was a

presence check in the property, which isn't as safe as a truthiness

check. While fine in practice, this issue was encountered when changing

Babel's TypeScript options to enable allowDeclaredFields, which

resulted in extraData being explicitly set to undefined.
The safer check and the asserts should help catch any confusing issues

in the future.

Unit tests pass.
Tested with the failure condition and saw the more useful error.