Vue.extend = function (extendOptions) {
  //***
  var Super = this;
  var SuperId = Super.cid;
  //***
  var Sub = function VueComponent(options) {
    this._init(options);
  };
  Sub.prototype = Object.create(Super.prototype);
  Sub.prototype.constructor = Sub;
  //***
  return Sub
};

Vue.nextTick().then(() => {
  // do sth
})

 if (typeof Promise !== 'undefined' && isNative(Promise)) {
 } else if (typeof MutationObserver !== 'undefined' && (
     isNative(MutationObserver) ||
     // PhantomJS and iOS 7.x
     MutationObserver.toString() === '[object MutationObserverConstructor]'
   )) {
 } else {
   // fallback to setTimeout
   /* istanbul ignore next */
   timerFunc = function () {
     setTimeout(nextTickHandler, 0);
   };
 }

<div id="app">
  <div ref="dom">{{a}}</div>
</div>
new Vue({
  el: '#app',
  data: {
    a: 1
  },
  mounted: function name(params) {
    console.log('start');
    this.$nextTick(function () {
      console.log('beforeChange', this.$refs.dom.textContent)
    })
    this.a = 2;
    console.log('change');
    this.$nextTick(function () {
      console.log('afterChange', this.$refs.dom.textContent)
    })
    console.log('end');
  }
})
// 控制臺(tái)依次打印
// start
// change
// end
// beforeChange 1
// afterChange 2

var ob = (target).__ob__;
if (target._isVue || (ob && ob.vmCount)) {
"development" !== 'production' && warn(
  'Avoid adding reactive properties to a Vue instance or its root $data ' +
  'at runtime - declare it upfront in the data option.'
);
return val
}

var a = [1, 2, 3];
delete a[0];
console.log(a); // [undefined, 2, 3]

Vue.use = function (plugin) {
  var installedPlugins = (this._installedPlugins || (this._installedPlugins = []));
  if (installedPlugins.indexOf(plugin) > -1) {
    return this
  }
  // additional parameters
  var args = toArray(arguments, 1);
  args.unshift(this);
  if (typeof plugin.install === 'function') {
    plugin.install.apply(plugin, args);
  } else if (typeof plugin === 'function') {
    plugin.apply(null, args);
  }
  installedPlugins.push(plugin);
  return this
};

Vue.use({
  a: 1,
  install: function (Vue) {
    console.log(this.a) // 1
    console.log(arguments) // [function Vue(options),"a", "b", "c"]
  }
}, 'a', 'b', 'c')

'use strict'
Vue.use(function plugin() {
  console.log(this) // null
  console.log(arguments) // [function Vue(options),"a", "b", "c"]
}, 'a', 'b', 'c')

var compiled = Vue.compile(
  '<div>' +
  '<header><h1>no data binding</h1></header>' +
  '<section>{{prop}}</section>' +
  '</div>'
)
console.log(compiled.render.toString())
console.log(compiled.staticRenderFns.toString())
// render
function anonymous() {
  with(this) {
    return _c('div', [_m(0), _c('section', [_v(_s(prop))])])
  }
}
// staticRenderFns
function anonymous() {
  with(this) {
    return _c('header', [_c('h1', [_v("no data binding")])])
  }
}

function anonymous() {
  with(this) {
    return _c('div', [_c('header', [_v("no data binding")]), _c('section', [_v(_s(prop))])])
  }
}

 // For a node to qualify as a static root, it should have children that
 // are not just static text. Otherwise the cost of hoisting out will
 // outweigh the benefits and it's better off to just always render it fresh.
 if (node.static && node.children.length && !(
     node.children.length === 1 &&
     node.children[0].type === 3
   )) {
   node.staticRoot = true;
   return
 } else {
   node.staticRoot = false;
 }

// internal render helpers.
// these are exposed on the instance prototype to reduce generated render
// code size.
Vue.prototype._o = markOnce;
Vue.prototype._n = toNumber;
Vue.prototype._s = toString;
Vue.prototype._l = renderList;
Vue.prototype._t = renderSlot;
Vue.prototype._q = looseEqual;
Vue.prototype._i = looseIndexOf;
Vue.prototype._m = renderStatic;
Vue.prototype._f = resolveFilter;
Vue.prototype._k = checkKeyCodes;
Vue.prototype._b = bindObjectProps;
Vue.prototype._v = createTextVNode;
Vue.prototype._e = createEmptyVNode;
Vue.prototype._u = resolveScopedSlots;
Vue.prototype._g = bindObjectListeners;