The Dart commit queue needs 10 more Mac machines in the Dart.LUCI pool
We just had to remove a mac builder from the CQ because the pool was
overcommitted, and pending jobs were timing out.
We have only 4 Macs which are 8-CPU VMs, and need more of these
faster macs to run this builder.  We got 6 Mac minis, which are cheaper,
but they turned out to be slower.  We are still using them on othe.r builders, though.

If these are hard to get, we could move this builder (and others) to Linux instead, and use 20 additional Linux VMs in the pool instead, identical to those already there.

See issue https://bugs.chromium.org/p/chromium/issues/detail?id=756472 for previously allocated machines.

We will start trying to use the CQ resource estimation process used by CIT for the chrome CQs now, to get a better picture of our needs in the future.