Currently we have `tsc` [generate code for es5], because we don't yet have es6 destructuring support in JSV (used [here], for example).
As a follow on from #2587, this disables the TS poiyfill and points it at our `Object.assign`, so we benefit from these new features, while still keeping the target at es5.
[generate code for es5]: 380af9c600/tsconfig.json (L4-L4)
[here]: 380af9c600/apps/btadv/app.ts (L235)