[dart-roll] Roll to a188781c9fc8ab950605716fb813bc5f2ca70e25.

[aam]

Changes since last roll:

a188781c9fc (tag: 2.12.0-28.0.dev, origin/dev) Version 2.12.0-28.0.dev
9d132ebba9f Bump pub
391bc8d33d6 [vm/concurrency] Allow calling SafepointRwLock::IsCurrentThreadReader() inside safepoint operation scope
466f2fd8a79 Analyzer: prepare some generated/ tests for null safety
e90967f29de [SDK] Makes --lazy_async_stack default.
6101e4653b5 (origin/lkgr) [test/ffi] Add struct stack alignment test

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[aam]

Made obsolete by #22375 and subsequent automated roll.