Avoid negatives in the styleguide.md (#172917)

Tell Gemini styleguide.md reference what to do instead of what not to
do.

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

**Note**: The Flutter team is currently trialing the use of [Gemini Code
Assist for
GitHub](https://developers.google.com/gemini-code-assist/docs/review-github-code).
Comments from the `gemini-code-assist` bot should not be taken as
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<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md
https://dart.googlesource.com/external/github.com/flutter/flutter/+/6a8969cecadc8cd3ae6fb90373a44aff6da8e57d
diff --git a/DEPS b/DEPS
index c81187b..d8f7e22 100644
--- a/DEPS
+++ b/DEPS
@@ -231,7 +231,7 @@
 deps = {
   'flutter/engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + 'c1c62259e786c00fe0090bd89f7a1bfa49b9b227',
 
-  'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + '60184bfbc139cd2c37a3d78eb570be8d84bd6959',
+  'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + '6a8969cecadc8cd3ae6fb90373a44aff6da8e57d',
 
   'flutter/engine/src/flutter/third_party/depot_tools':
   Var('chromium_git') + '/chromium/tools/depot_tools.git' + '@' + '580b4ff3f5cd0dcaa2eacda28cefe0f45320e8f7',
diff --git a/commits.json b/commits.json
index d32fb0a..6bd8a80 100644
--- a/commits.json
+++ b/commits.json
@@ -1,4 +1,4 @@
 {
-  "flutter":"60184bfbc139cd2c37a3d78eb570be8d84bd6959",
+  "flutter":"6a8969cecadc8cd3ae6fb90373a44aff6da8e57d",
   "flutter/engine/src/flutter/third_party/dart":"c1c62259e786c00fe0090bd89f7a1bfa49b9b227"
 }
\ No newline at end of file