Upgrade to 3.7

Change-Id: Ic04af45ef5cda5f721206b6edd630c3bd0c48277
Reviewed-on: https://dart-review.googlesource.com/c/deps/+/409101
Reviewed-by: Ivan Inozemtsev <iinozemtsev@google.com>
Commit-Queue: Alexander Thomas <athom@google.com>
Commit-Queue: Ivan Inozemtsev <iinozemtsev@google.com>
Auto-Submit: Alexander Thomas <athom@google.com>