Function copyAugment

vue.js:1421–1426

(target, src, keys)

     */
    /* istanbul ignore next */
    function copyAugment(target, src, keys) {
        for (var i = 0, l = keys.length; i < l; i++) {
            var key = keys[i];
            def(target, key, src[key]);
        }
    }

    /**
     * Attempt to create an observer instance for a value,