Bump pub to 400f21e9883ce6555b66d3ef82f0b732ba9b9fc8
Bump pub to 400f21e9883ce6555b66d3ef82f0b732ba9b9fc8

Changes:
```
> git log --format="%C(auto) %h %s" 35681b0..400f21e
 https://dart.googlesource.com/pub.git/+/400f21e9 migrate pub client (except tests) to null-safety (#3178)
 https://dart.googlesource.com/pub.git/+/909852fc Remove unused `pub run --dart-dev` (#3177)

```

Diff: https://dart.googlesource.com/pub.git/+/35681b0126a1fb48bf2062dd09f74296715402c2~..400f21e9883ce6555b66d3ef82f0b732ba9b9fc8/
Change-Id: If428fda768156f8923be3d739e80b9ac9a37eb87
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/217188
Auto-Submit: Jonas Jensen <c9327fa216ae2ef84df7c55b809d4ba270141b48@google.com>
Commit-Queue: Alexander Thomas <29642742b6693024c89de8232f2e2542cf7eedf7@google.com>
Reviewed-by: Alexander Thomas <29642742b6693024c89de8232f2e2542cf7eedf7@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk