[Cloud Firestore] Unsupported operation: Platform._operatingSystem

I keep getting this with `web-enabled`:

```
══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
The following UnsupportedError was thrown building HomePage(dirty):
Unsupported operation: Platform._operatingSystem

User-created ancestor of the error-causing widget was:
Scaffold org-dartlang-app:///packages/x/main.dart:25:15

```