code: function registerUpgradedCallbackInternal(jsClass, callback) {
      var regClass = findRegisteredClass_(jsClass);
      if (regClass) {
        regClass.callbacks.push(callback);
      }
    }
docs: Allows user to be alerted to any upgrades that are performed for a given
  component type
  
  @param {string} jsClass The class name of the MDL component we wish
  to hook into for any upgrades performed.
  @param {function(!HTMLElement)} callback The function to call upon an
  upgrade. This function should expect 1 parameter - the HTMLElement which
  got upgraded.