Started handling duplicate validation layer messages (#156375)

fixes https://github.com/flutter/flutter/issues/151528

Those tests were failing because the validation layers messages were
printing out twice. This now only starts failing if a backend is
reported that is not vulkan with validation layers. See
https://github.com/flutter/flutter/issues/151528#issuecomment-2398189205

## 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
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[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/+/5c6e3f03c0ba13002e05e34d05bd1ee51c75bbfe
diff --git a/DEPS b/DEPS
index 0c77371..e2a1dab 100644
--- a/DEPS
+++ b/DEPS
@@ -284,7 +284,7 @@
 
   'engine/src/flutter/third_party/dart': Var('dart_git') + '/sdk' + '@' + '91cce5d0f0a2817241286c3938badb8d9d0f8aba',
 
-  'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + '0baf7a59041478e0d2958106b957e70437cb28f5',
+  'flutter': Var('flutter_git') + '/mirrors/flutter' + '@' + '5c6e3f03c0ba13002e05e34d05bd1ee51c75bbfe',
 
   '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 80977d9..0acd31d 100644
--- a/commits.json
+++ b/commits.json
@@ -1,5 +1,5 @@
 {
   "engine/src/flutter":"4a97943e055642247a054c83e2c6933e40863475",
   "engine/src/flutter/third_party/dart":"91cce5d0f0a2817241286c3938badb8d9d0f8aba",
-  "flutter":"0baf7a59041478e0d2958106b957e70437cb28f5"
+  "flutter":"5c6e3f03c0ba13002e05e34d05bd1ee51c75bbfe"
 }
\ No newline at end of file