refactor(*): replace `HashMap` with `NgMap`

For the time being, we will be using `NgMap`, which is an API-compatible
implementation of native `Map` (for the features required in Angular). This will
make it easy to switch to using the native implementations, once they become
more stable.

Note:
At the moment some native implementations are still buggy (often in subtle ways)
and can cause hard-to-debug failures.)

Closes #15483