The file `source.ts` contains a TypeScript program that exposes a running
histogram through a view interface: consumers may read the running count and
max, and push new bucket indices in, but never read the bucket back through
that view. The concrete aggregator behind the view stores its bucket in a
wider slot so it can also hold named sentinels from other producers.

Convert it to idiomatic Flow in `main.js`, preserving the runtime behavior.
The aggregator must be usable through the view interface without any casts.
