Bump core to dc975303c312497aec2f5ed4c5828da3607e33dd

Changes:
```
> git log --format="%C(auto) %h %s" af37fe5..dc97530
 https://dart.googlesource.com/core.git/+/dc975303 Speedup package:crypto (with a focus on md5) (892)
 https://dart.googlesource.com/core.git/+/66348be6 Remove munificent as owner of args in CODEOWNERS (891)
 https://dart.googlesource.com/core.git/+/816cea7c [path] Fix test race condition caused by changing Directory.current (890)
 https://dart.googlesource.com/core.git/+/de09be6d update the issue templates for package:lints (887)
 https://dart.googlesource.com/core.git/+/f608a009 Format packages to new style (883)
 https://dart.googlesource.com/core.git/+/804ff112 Fix doc comment issues reported by pub.dev (882)
 https://dart.googlesource.com/core.git/+/c064ca85 Add `no_response.yaml` workflow (881)
 https://dart.googlesource.com/core.git/+/228112c0 prep to publish 6.0.0 of package:lints (878)
 https://dart.googlesource.com/core.git/+/e32c7b96 Fix StreamGroup.broadcast() close() not completing when streams close. (876)

```

Diff: https://dart.googlesource.com/core.git/+/af37fe54c320b0f76f76da01acbbfc0c0118e613..dc975303c312497aec2f5ed4c5828da3607e33dd/
Change-Id: Ib9fa5862fc3e354b440c39b77e047d04bbe4d128
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/434160
Commit-Queue: Konstantin Shcheglov <[email protected]>
Reviewed-by: Konstantin Shcheglov <[email protected]>
Auto-Submit: Devon Carew <[email protected]>

Author: devoncarew

DEPS

@@ -130,7 +130,7 @@ vars = {
   # 'tools/rev_sdk_deps.dart' will rev pkg dependencies to their latest; put an
   # EOL comment after a dependency to instead pin at the current revision.
   "ai_rev": "f2b48c6bf688e27f41b0b73ee3ff0164a05da157",
-  "core_rev": "af37fe54c320b0f76f76da01acbbfc0c0118e613",
+  "core_rev": "dc975303c312497aec2f5ed4c5828da3607e33dd",
   "dartdoc_rev": "4ceea6b8240bf1dd9694a170368264e40c67d66b",
   "ecosystem_rev": "64aac3a9c4606950bcf6c8729f01ed8548b9ed87",
   "flute_rev": "e4ea0459a7debae5e9592c85141707b01fac86c9",