Bump pub to a3a102a549388a6dbfecc9252fabb618f9a2f5f7

Changes:
```
> git log --format="%C(auto) %h %s" 8f5ab7b..a3a102a
 https://dart.googlesource.com/pub.git/+/a3a102a5 Fix equality and hashcode for the sdk descriptors (#3367)
 https://dart.googlesource.com/pub.git/+/94ae66a6 Refine what a relative uri means in a git path (#3212)
 https://dart.googlesource.com/pub.git/+/cc4c1292 Only call Package.listFiles once per publish. (#3346)
 https://dart.googlesource.com/pub.git/+/f4484073 Fix test/global/activate/git_package_test test on windows (#3361)
 https://dart.googlesource.com/pub.git/+/610ce7f2 Refactor descriptors (#3305)
 https://dart.googlesource.com/pub.git/+/953b6097 Substitute pub.dartlang.org for of pub.dev (#3358)
 https://dart.googlesource.com/pub.git/+/7a6ea396 Add support for pubspec overrides file (#3215)
 https://dart.googlesource.com/pub.git/+/8abfed9d Global activate git path and ref (#3356)
 https://dart.googlesource.com/pub.git/+/d1c0e3f9 Revert "Add flag controlling creation of `.packages` file. (#2757)" (#3357)
 https://dart.googlesource.com/pub.git/+/274f5ad9 Fix signals test (#3359)
 https://dart.googlesource.com/pub.git/+/83437005 Avoid failing in gitignore validator (#3354)
 https://dart.googlesource.com/pub.git/+/3082796f dependency_services: Don't download archives on apply (#3352)
 https://dart.googlesource.com/pub.git/+/48d0ffaf dependency_services: Use ^ constraints for widened intervals when possible (#3349)
 https://dart.googlesource.com/pub.git/+/826e2086 Remove obsolete test (#3347)
 https://dart.googlesource.com/pub.git/+/35e5140b Bump analyzer from 2.8.0 to 3.3.1 (#3341)
 https://dart.googlesource.com/pub.git/+/52f2bdc2 Enable dependabot (#3340)
 https://dart.googlesource.com/pub.git/+/1e70c0c7 Remove `uploader` command (#3335)
 https://dart.googlesource.com/pub.git/+/3174a264 Warn if git version is not high enough for supporting all features (#3332)
 https://dart.googlesource.com/pub.git/+/3f7a3cb7 Don't analyze ignored directories in directory-validator (#3331)
 https://dart.googlesource.com/pub.git/+/e8f36614 Allow use of token for talking to pub.dev (#3330)
 https://dart.googlesource.com/pub.git/+/b93bf88f Upgrade `package:tar` to version `0.5.4`. (#3313)
 https://dart.googlesource.com/pub.git/+/fbc9732e Support for different versioning strategies in dependency_services (#3320)
 https://dart.googlesource.com/pub.git/+/93c7cfcd Update repository-spec-v2.md (#3311)
 https://dart.googlesource.com/pub.git/+/941191f7 dependency_services (#3304)
 https://dart.googlesource.com/pub.git/+/61175cb6 fix: relative to the current directory rules (#3297)
 https://dart.googlesource.com/pub.git/+/f27e90d3 Upgrade other versions conservatively with --major-versions (#3295)
 https://dart.googlesource.com/pub.git/+/a2461417 Add flag controlling creation of `.packages` file. (#2757)

```

Diff: https://dart.googlesource.com/pub.git/+/8f5ab7b1aba3b9f66b56246d77e167990339d317~..a3a102a549388a6dbfecc9252fabb618f9a2f5f7/
Change-Id: I8d0ea375039ea450d397871d9fac35d590ea8869
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/239315
Reviewed-by: Jonas Jensen <jonasfj@google.com>
Commit-Queue: Sigurd Meldgaard <sigurdm@google.com>

CHANGELOG.md

@@ -233,8 +233,16 @@ in 2018, as it doesn't work with any Dart 2.x release.
   tracker.
 
   `dart --verbose pub [command]` will also cause the log file to be written.
+- `dart pub global activate --source=git` now takes arguments `--git-path` to
+  specify the path of the activated package in the pubspec and `--git-ref` to
+  specify the branch or revision to check out.
 - `dart pub add` can now add multiple packages in one command.
-
+- `dart pub token add` can now add a token for [pub.dev](https://pub.dev).
+- `dart pub uploader` has been removed. To manage uploaders for a package use
+  the `https://pub.dev/<packagename>/admin` web-interface.
+- Pub now supports a separate `pubspec_overrides.yaml` file that can contain
+  `dependency_overrides`. This makes it easier to avoid checking the local
+  overrides into version control.
 #### Linter
 
 Updated the Linter to `1.18.0`, which includes changes that

DEPS

@@ -141,7 +141,7 @@ vars = {
   "pool_rev": "7abe634002a1ba8a0928eded086062f1307ccfae",
   "process_rev": "56ece43b53b64c63ae51ec184b76bd5360c28d0b",
   "protobuf_rev": "c1eb6cb51af39ccbaa1a8e19349546586a5c8e31",
-  "pub_rev": "8f5ab7b1aba3b9f66b56246d77e167990339d317",
+  "pub_rev": "a3a102a549388a6dbfecc9252fabb618f9a2f5f7",
   "pub_semver_rev": "ea6c54019948dc03042c595ce9413e17aaf7aa38",
   "root_certificates_rev": "692f6d6488af68e0121317a9c2c9eb393eb0ee50",
   "rust_revision": "b7856f695d65a8ebc846754f97d15814bcb1c244",