[dart] Do not pass --cleanup-dart-processes on stable

The flag does not exist on stable, yet. We can revert this when 2.12 has
been merged to stable.

Change-Id: I2061ed989befa6c5845b348bb23d3d96cdb1084d
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/171562
Reviewed-by: William Hesse <whesse@google.com>
Commit-Queue: Alexander Thomas <athom@google.com>
2 files changed