Use case

zircon.System in flutter engine defines a clockGet function that uses the Fuchsia zx_clock_get syscall and the associated concept of a clock ID.

Per RFC-0008 this syscall and the concept of a clock ID are deprecated in Fuchsia and I'm working through the long process of removing dependencies, including Flutter.

Proposal

Thankfully this function is not heavily used and all the invocations I can find are for monotonic time. Therefore, as documented in fxbug.dev/66266 I'm recommending the following:

1. Add a new clockGetMonotonic operation in zircon.System.
2. Wait for that to roll into fuchsia and other repos
3. Move all the call sites from zircon.System.clockGet(_zxClockMonotonic) to zircon.System.clockGetMonotonic()
4. Mark the clockGet function deprecated upstream (and maybe have it return an exception instead of attempting the syscall so we unblock the removal of the syscall?)
5. Wait some more
6. Remove zircon.System.clockGet entirely