[dart] Remove obsolete browser fallback on test-matrix versions

Change-Id: I6fed7fd166e442a6a76f4c521cabf81af895f7ee
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/203620
Reviewed-by: Karl Klose <karlklose@google.com>
Commit-Queue: Alexander Thomas <athom@google.com>
19 files changed