Fix warning about this being undefined

[saraedum]

When running yarn run build we get:

(!) `this` has been rewritten to `undefined`
https://rollupjs.org/guide/en/#error-this-is-undefined
node_modules/vue-property-decorator/lib/decorators/Emit.js
1: var __spreadArrays = (this && this.__spreadArrays) || function () {
                         ^
2:     for (var s = 0, i = 0, il = arguments.length; i < il; i++) s += arguments[i].length;
3:     for (var r = Array(s), k = 0, i = 0; i < il; i++)
...and 1 other occurrence