Revert "[dart] Support try-builds without a gerrit change"

This reverts commit 36a5989b530a162852d1e9da13dd23cb702dd787.

Reason for revert: Speculative revert because of purple tryjobs.

Original change's description:
> [dart] Support try-builds without a gerrit change
>
> This will avoid the infra failures when running try-builds with led.
>
> Change-Id: I6557103ca59c8f5f26f18327abd1e135dbe2a8c0
> Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/194700
> Commit-Queue: Karl Klose <karlklose@google.com>
> Reviewed-by: Alexander Thomas <athom@google.com>

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

Change-Id: I6b197b429dc7ffc97dd09762f37f9f8b97776adb
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/194780
Reviewed-by: Alexander Thomas <athom@google.com>
Commit-Queue: Alexander Thomas <athom@google.com>
16 files changed