code: function processState (instance) {
    const props = isLegacy
      ? instance._props
      : instance.$options.props
    const getters =
      instance.$options.vuex &&
      instance.$options.vuex.getters
    return Object.keys(instance._data)
      .filter(key => (
        !(props && key in props) &&
        !(getters && key in getters)
      ))
      .map(key => ({
        key,
        value: instance._data[key],
        editable: true
      }))
  }
docs: Process state, filtering out props and "clean" the result
  with a JSON dance. This removes functions which can cause
  errors during structured clone used by window.postMessage.
  
  @param {Vue} instance
  @return {Array}