Remove private fields from mixins

[kerwanp]

Description

Private properties are not allowed in mixins and private fields (ex: #create()) result in a strange behavior.

Proposal

Mixins should never have any private field. They must be either protected or public and when the usage is "internal only" they must be prefixed with $.

class ManagesCustomer {
  publicMethod() {}
  $internalMethod() ()
}