Bump pub to 51435efcd574b7bc18d47a5dd620cb9759dea8f8

Changes:
```
> git log --format="%C(auto) %h %s" 1e3c17e..51435ef
 https://dart.googlesource.com/pub.git/+/51435efc Reland "Add flag controlling creation of `.packages` file." (#3413)

```

Diff: https://dart.googlesource.com/pub.git/+/1e3c17ea871e6a80c720aa998f37cbd3913bc287~..51435efcd574b7bc18d47a5dd620cb9759dea8f8/
Change-Id: Ic6e28328827cf9aada1272decf4d95fc6a649c90
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/244623
Commit-Queue: Sigurd Meldgaard <sigurdm@google.com>
Reviewed-by: Jonas Jensen <jonasfj@google.com>

CHANGELOG.md

@@ -30,6 +30,9 @@ Updated the Linter to `1.23.0`, which includes changes that
 
 #### Pub
 
+* Breaking: `dart pub get` and `dart pub upgrade` no longer creates the
+  [deprecated](https://github.com/dart-lang/sdk/issues/47431) `.packages` file.
+  It can still be created with the `--legacy-packages-file` flag.
 * `dart pub outdated` now shows which of your dependencies are discontinued.
 
 ## 2.17.0

DEPS

@@ -138,7 +138,7 @@ vars = {
   "pool_rev": "7abe634002a1ba8a0928eded086062f1307ccfae",
   "process_rev": "56ece43b53b64c63ae51ec184b76bd5360c28d0b",
   "protobuf_rev": "c1eb6cb51af39ccbaa1a8e19349546586a5c8e31",
-  "pub_rev": "1e3c17ea871e6a80c720aa998f37cbd3913bc287",
+  "pub_rev": "51435efcd574b7bc18d47a5dd620cb9759dea8f8",
   "pub_semver_rev": "ea6c54019948dc03042c595ce9413e17aaf7aa38",
   "root_certificates_rev": "692f6d6488af68e0121317a9c2c9eb393eb0ee50",
   "rust_revision": "b7856f695d65a8ebc846754f97d15814bcb1c244",