* Force depot_tools to use python3 results (USE_PYTHON3=True).
* Fixes the dart format presubmit check.
* Remove broken DOM tools presubmit check.
TEST=Manually provoked errors and ran git cl presubmit -v -f.
Cq-Include-Trybots: luci.dart.try.shared:presubmit-try
Change-Id: I8ba46e2ae1640f1b2f82e18bc8024e0aa4838b2b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/210123
Reviewed-by: Ben Konyi <bkonyi@google.com>
Reviewed-by: William Hesse <whesse@google.com>