[co19] Roll co19 to 6f34e53fa54396f17bcffcd307528f714a099e03

2024-03-05 sgrekhov22@gmail.com dart-lang/co19#2559. Add tests checking that `augment` is a built-in identifier (dart-lang/co19#2560)
2024-03-01 49699333+dependabot[bot]@users.noreply.github.com Bump actions/setup-java from 4.0.0 to 4.1.0 (dart-lang/co19#2557)
2024-02-23 sgrekhov22@gmail.com dart-lang/co19#2548. Add tests for out parameter in a contrvariant position (dart-lang/co19#2555)
2024-02-22 sgrekhov22@gmail.com dart-lang/co19#2548. Add variance modifier declaration tests (dart-lang/co19#2554)
2024-02-21 sgrekhov22@gmail.com dart-lang/co19#2548. Add `out` and `inout` to built-in identifiers tests (dart-lang/co19#2553)

Change-Id: Idd8ddf54f4cd7bb04b7296068653bee336d6d193
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/356460
Reviewed-by: Erik Ernst <eernst@google.com>
Reviewed-by: Alexander Thomas <athom@google.com>

DEPS

@@ -54,7 +54,7 @@ vars = {
 
   # co19 is a cipd package automatically generated for each co19 commit.
   # Use tests/co19/update.sh to update this hash.
-  "co19_rev": "867d139b3169fc131488e893ec1133dc98cc3aa0",
+  "co19_rev": "6f34e53fa54396f17bcffcd307528f714a099e03",
 
   # The internal benchmarks to use. See go/dart-benchmarks-internal
   "benchmarks_internal_rev": "a7c23b2422492dcc515d1ba4abe3609b50e2a139",