initChildAddons(addonInstance) {
    if (addonInstance.initializeAddons) {
      addonInstance.initializeAddons();
    } else {
      addonInstance.addons = [];
    }
  }
Initialize the child addons array of a newly-created addon instance. Normally when
an addon derives from Addon, child addons will be created during 'setupRegistry' and
this code is essentially unnecessary. But if an addon is created with custom constructors
that don't call 'setupRegistry', any child addons may not yet be initialized.

@method initChildAddons
@param {Addon} addonInstance