code: function(handle) {
          if (isValidID(handle)) {
            var deferred = $q.defer();
            var instance = self.get(handle);
  
            if (instance)  {
              deferred.resolve(instance);
            } else {
              if (pendings[handle] === undefined) {
                pendings[handle] = [];
              }
              pendings[handle].push(deferred);
            }
  
            return deferred.promise;
          }
          return $q.reject("Invalid `md-component-id` value.");
        }
docs: Async accessor to registered component instance
  If not available then a promise is created to notify
  all listeners when the instance is registered.