[dart] Do not mark SDK bisection builds as experimental

Change-Id: I16e88774d9ac7bde998a0f6e0d61c8d25b314bd2
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/157761
Commit-Queue: Karl Klose <karlklose@google.com>
Reviewed-by: Alexander Thomas <athom@google.com>
7 files changed