vue源碼的疑惑

無意中看見vue源碼

export function initMixin (Vue: Class<Component>) { 
    Vue.prototype._init = function (options?: Object) {
    const vm: Component = this
    // a uid
    vm._uid = uid++

    ...
}

里面好多Vue: Class<Component> 和 const vm: Component = this，這些都是什么語法？