* Migrate to python3; drop python support.
* Update Windows toolchain support.
* Remove some unused methods.
* Python 2.7 is still needed on Windows.
* Update gsutil to a version that supports python3.
Fixes: https://github.com/dart-lang/sdk/issues/28793
TEST=Manually tested common user journeys.
Change-Id: I663a22b237a548bb82dc2e601e399e3bc3649211
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/192182
Reviewed-by: William Hesse <whesse@google.com>
Reviewed-by: Alexander Aprelev <aam@google.com>
Readd samples, samples-dev, tools/testing/bin, third_party removed by cleanup (no history, sorry)
Add DEPS file which will replace the old all.deps and standalone.deps
Add tools/deps/dartium.deps replacing the old dartium.deps (but now editable from a normal checkout)
Fixup tools/utils.py to use the new archiving schema (git count for be, version number for dev/stable
Fix codereview.settings
Alexander Thomas