主要相关文件：instance/index.js、instance/init.js、instance/state.js、instance/events.js、instance/lifecycle.js、instance/

function Vue (options) {
  if (process.env.NODE_ENV !== 'production' &&
    !(this instanceof Vue)
  ) {
    warn('Vue is a constructor and should be called with the `new` keyword')
  }
  this._init(options)
}
initMixin(Vue)
stateMixin(Vue)
eventsMixin(Vue)
lifecycleMixin(Vue)
renderMixin(Vue)

initMixin

1. 安装 Vue 初始化函数 _init

stateMinxin

1. 设置 $data 和 $props 的访问代理
   1. $data -> _data
   2. $props -> _props
2. 安装数据相关的实例函数
   1. $set
   2. $delete
   3. $watch

eventMinxin

1. 安装事件相关的实例方法
   1. $on
   2. $once
   3. $off
   4. $emit

lifecycleMixin

1. 安装视图更新函数 _update
2. 安装生命周期相关的实例方法
   1. $forceUpdate
   2. [$destroy](https://cn.vuejs.org/v2/api/#vm-destroy)

renderMixin

1. 利用 installRenderHelpers 安装了运行时所需的工具函数

   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

2. 安装渲染所需的实例函数

   1. $nextTick
   2. _render

总结

// initMixin(Vue)    src/core/instance/init.js **************************************************
Vue.prototype._init = function (options?: Object) {}
// stateMixin(Vue)    src/core/instance/state.js **************************************************
Vue.prototype.$data
Vue.prototype.$props
Vue.prototype.$set = set
Vue.prototype.$delete = del
Vue.prototype.$watch = function (
  expOrFn: string | Function,
  cb: any,
  options?: Object
): Function {}
// eventsMixin(Vue)    src/core/instance/events.js **************************************************
Vue.prototype.$on = function (event: string | Array<string>, fn: Function): Component {}
Vue.prototype.$once = function (event: string, fn: Function): Component {}
Vue.prototype.$off = function (event?: string | Array<string>, fn?: Function): Component {}
Vue.prototype.$emit = function (event: string): Component {}
// lifecycleMixin(Vue)    src/core/instance/lifecycle.js **************************************************
Vue.prototype._update = function (vnode: VNode, hydrating?: boolean) {}
Vue.prototype.$forceUpdate = function () {}
Vue.prototype.$destroy = function () {}
// renderMixin(Vue)    src/core/instance/render.js **************************************************
// installRenderHelpers 函数中
Vue.prototype._o = markOnce
Vue.prototype._n = toNumber
Vue.prototype._s = toString
Vue.prototype._l = renderList
Vue.prototype._t = renderSlot
Vue.prototype._q = looseEqual
Vue.prototype._i = looseIndexOf
Vue.prototype._m = renderStatic
Vue.prototype._f = resolveFilter
Vue.prototype._k = checkKeyCodes
Vue.prototype._b = bindObjectProps
Vue.prototype._v = createTextVNode
Vue.prototype._e = createEmptyVNode
Vue.prototype._u = resolveScopedSlots
Vue.prototype._g = bindObjectListeners
Vue.prototype.$nextTick = function (fn: Function) {}
Vue.prototype._render = function (): VNode {}
// core/index.js 文件中
Object.defineProperty(Vue.prototype, '$isServer', {
  get: isServerRendering
})
Object.defineProperty(Vue.prototype, '$ssrContext', {
  get () {
    /* istanbul ignore next */
    return this.$vnode && this.$vnode.ssrContext
  }
})
// 在 runtime/index.js 文件中
Vue.prototype.__patch__ = inBrowser ? patch : noop
Vue.prototype.$mount = function (
  el?: string | Element,
  hydrating?: boolean
): Component {
  el = el && inBrowser ? query(el) : undefined
  return mountComponent(this, el, hydrating)
}
// 在入口文件 entry-runtime-with-compiler.js 中重写了 Vue.prototype.$mount 方法
Vue.prototype.$mount = function (
  el?: string | Element,
  hydrating?: boolean
): Component {
  // ... 函数体
}