Computed method is not setup if method does not exist at the point the mixin runs

[mrfelton]

Similar to fullcube/loopback-ds-calculated-mixin#7

Mixins run very early in the boot process - before components have run. Components can add properties and methods to models. If you set up this mixin that uses a Model function that is defined in a component (and therefore after the mixin initialization has run), the mixin will not be setup properly.