Bump pub to dcb6abac2d7d43258c03b348be42bf4aab9529b1

Changes:
```
> git log --format="%C(auto) %h %s" b9edfa5..dcb6aba
 https://dart.googlesource.com/pub.git/+/dcb6abac Merge remote-tracking branch 'origin/cherry-pick2-for-2.15.0'
 https://dart.googlesource.com/pub.git/+/dc857523 Remove duplicated lines in testdata (#3234)
 https://dart.googlesource.com/pub.git/+/acc8ab09 Refactor the test package-server (#3230)
 https://dart.googlesource.com/pub.git/+/1e78c688 Better error messages round 2 (#3223)
 https://dart.googlesource.com/pub.git/+/eaf36513 More links in the repository specification (#3220)
 https://dart.googlesource.com/pub.git/+/c888b018 Remove pedantic and cleanup a few lints (#3224)
 https://dart.googlesource.com/pub.git/+/a4d44c7e Global package server null safety (#3225)
 https://dart.googlesource.com/pub.git/+/ff941887 Merge branch 'cherry-pick-for-2.15.0'
 https://dart.googlesource.com/pub.git/+/efd24e64 Fix hanging event handler for `stdin`. (#3218)
 https://dart.googlesource.com/pub.git/+/d77c14e8 Gitignore validator should not follow symlink dirs (#3209)
 https://dart.googlesource.com/pub.git/+/7c190789 migrate rest of test/ to null-safety (#3207)
 https://dart.googlesource.com/pub.git/+/f24adb64 migrate test/oath2/ test/outdated/ test/token test/validator/ to null-safety (#3206)
 https://dart.googlesource.com/pub.git/+/e7d77a57 migrate test/add, test/cache, test/embedding plus a bit more to null-safety (#3205)
 https://dart.googlesource.com/pub.git/+/a6a73ad2 migrate test/lish/, test/run/, test/upgrade/ and golden_file.dart to null-safety (#3204)
 https://dart.googlesource.com/pub.git/+/b90efc1f Add test that `pub get` works with no $HOME in environment. (#3173)
 https://dart.googlesource.com/pub.git/+/c2fe3966 migrate test/get/ to null-safety (#3203)
 https://dart.googlesource.com/pub.git/+/1d106898 migrate test/hosted/ to null-safety (#3200)
 https://dart.googlesource.com/pub.git/+/afa9932b migrate test/cache and test/downgrade/ to null-safety (#3201)
 https://dart.googlesource.com/pub.git/+/352ca989 migrate test/global/ to null-safety (#3202)
 https://dart.googlesource.com/pub.git/+/64f20ca4 migrate test/descriptor/ and couple more files in test/ to null-safety (#3192)

```

Diff: https://dart.googlesource.com/pub.git/+/b9edfa5e288ea3d1a57d1db054ef844ae7b27d99~..dcb6abac2d7d43258c03b348be42bf4aab9529b1/
Change-Id: I30d33b0b8c3c19097a16543db15c6ae633f0afc2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/221633
Auto-Submit: Jonas Jensen <jonasfj@google.com>
Reviewed-by: Alexander Thomas <athom@google.com>
Commit-Queue: Jonas Jensen <jonasfj@google.com>

DEPS

@@ -140,7 +140,7 @@ vars = {
   "pool_rev": "7abe634002a1ba8a0928eded086062f1307ccfae",
   "process_rev": "56ece43b53b64c63ae51ec184b76bd5360c28d0b",
   "protobuf_rev": "c1eb6cb51af39ccbaa1a8e19349546586a5c8e31",
-  "pub_rev": "b9edfa5e288ea3d1a57d1db054ef844ae7b27d99",
+  "pub_rev": "dcb6abac2d7d43258c03b348be42bf4aab9529b1",
   "pub_semver_rev": "a43ad72fb6b7869607581b5fedcb186d1e74276a",
   "root_certificates_rev": "692f6d6488af68e0121317a9c2c9eb393eb0ee50",
   "rust_revision": "b7856f695d65a8ebc846754f97d15814bcb1c244",