Reland "[dart] Use CAS backend instead of isolate server"

This reverts commit b76a53c9b401e5a07772d3d9a04af587bf351fc1 and
fixes a bug in the regular expression to exclude xcode on Mac builds.

Change-Id: I9525d2b50e65827c8f15defd593cd678afb08b1c
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/196081
Reviewed-by: Alexander Thomas <athom@google.com>
Commit-Queue: Karl Klose <karlklose@google.com>
12 files changed