ClassLoader/ModelFileDAO: Support for custom classes and facets

[mdittmer]

The ClassLoader and *ModelFileDAO implementations do not currently deal appropriately with custom property classes or of:s. Here's the issue:

• The current design walks axioms (including requires) on models it find()s from the ModelFileDAO
• This requires ModelFileDAO to instantiate models (not just pass their plain-object spec up to the ClassLoader)
• However, when ModelFileDAO invokes foam.core.Model.create(clsDef), it eventually gets around to adapting the properties AxiomArray; while adapting elements that have a class: that's not loaded yet, an error is thrown

It looks like properties, and anything else that can be specified with a custom class: need:

• A way to add a new class-load-promise to the current chain of promises to be resolved on ModelFileDAO.find()
• Some sort of upgrade() interface that allows placeholders to be upgraded. (Perhaps the AxiomArray controls the upgrade, and knows which placeholder to swap out?)

This allows the ModelFileDAO to return a model only after all its constituent parts are ready, and deals with the fact that the code "realizes" that the class is missing in the middle of synchronous Model.create().

[adamvy]

This could also be handled by async json parsing.

If the JSON parser had an async interface, it could pause whenever it encountered a class that wasn't available yet.