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

This reverts commit 3053a2918d8b61e83990d2b09e3f9cbe72063845.

Reason for revert: Failures on mac-debug

Original change's description:
> [dart] Use CAS backend instead of isolate server
>
> Change-Id: I2da2b5c0bfea1dba01b13c2f978350042dc1e1bc
> Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/195120
> Commit-Queue: Karl Klose <karlklose@google.com>
> Reviewed-by: Alexander Thomas <athom@google.com>

TBR=karlklose@google.com,athom@google.com

Change-Id: I41982f4e7016296e41f34c85d1eb077f2791733c
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/196080
Reviewed-by: Karl Klose <karlklose@google.com>
Commit-Queue: Karl Klose <karlklose@google.com>
12 files changed