Fixes #1026.
Fixes #1026.

Does  not report for subclasses of nil created purposely, as they use
JS null to denote superclass.

The warning appears only for undefined (fixes loading .js)
and/or nil (fixes importing with missing global).

adrian-castravete/amber,amber-smalltalk/amber,amber-smalltalk/amber,amber-smalltalk/amber,adrian-castravete/amber,adrian-castravete/amber