fix: should prefix `ShadowRoot` with `window.` (#2943)
Otherwise this expression would throw in environments that does not
support `ShadowRoot` which includes the common mocha testing environment
setup that uses `jsdom` and `jsdom-global`.
It is because `ShadowRoot` is not an enumerable property on `window`,
`jsdom-global` fails to expose it on the `global` object.
See the error message at: https://app.circleci.com/pipelines/github/vuejs/vue-cli/779/workflows/17d7d7c4-7605-4588-878a-ddb3a6d37102/jobs/24147
Haoqun Jiang