[dart] Add support for the new experimental channel

* -experimental builders are treated as if "is_experimental" is true.
* Unify channel detection logic.

Bug: https://crbug.com/1207358
Change-Id: I8a11e3b3d1f9913a060e7b8757dc727d7ad79103
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/201420
Reviewed-by: Karl Klose <karlklose@google.com>
Commit-Queue: Alexander Thomas <athom@google.com>
6 files changed