code: function runModelObservers(proc) {
    const observersIterator = (obs) => obs();
    maybeForEach(proc.observers, observersIterator);
    if (proc.parent) {
      maybeForEach(proc.childrenObservers, observersIterator);
    }
  }
docs: Ren all model observers and returns a Promise which will
  be resolved when all handlers executed and returned promises
  is also resolved
  
  @private
  @param  {Process} proc
  @return {Promise}