fix #354 test failure

[Rich-Harris]

#354

[Rich-Harris]

There's something weird going on here — it's failing to build svelte-kit-demo

[benmccann]

I can confirm the tests pass on my old machine with this change

[benmccann]

Running npm run dev and then visiting localhost:3000 on the svelte-kit-demo gives:

TypeError: Response is not a constructor
    at fetcher (packages/kit/dist/ssr.js:2075:17)
    at async load (/home/bmccann/src/kit/examples/svelte-kit-demo/src/routes/blog/index.svelte:5:11)
    at async get_response (packages/kit/dist/ssr.js:2138:6)
    at async render_page (packages/kit/dist/ssr.js:2344:9)
    at async ssr (packages/kit/dist/ssr.js:2482:20)
    at async Immediate.<anonymous> (packages/kit/dist/chunks/index.js:3278:23)