code: function update(vm) {
          var i = vm._watchers.length
          while (i--) {
              vm._watchers[i].update(true); // shallow updates
          }
          var children = vm.$children;
          i = children.length;
          while (i--) {
              var child = children[i]
              update(child)
          }
      }
docs: Credits: @Haixing-Hu
  
  Updates all the watchers in the Vue instance of a component tree.
  
  This function is inspired by the "_digest()" function in the
  "src/instance/scope.js" of the source of Vue.js, excepts that this function
  updates the children components no matter whether it is inheritable.
  
  @param vm
  the root of the component tree.