[gardening] Mark pkg/plugin_manager_test as flaky.

Bug: https://github.com/dart-lang/sdk/issues/34231

Change-Id: Iee983cebb48b0dfab97282a6f11f07cb2a18617a
Reviewed-on: https://dart-review.googlesource.com/71224
Reviewed-by: Alexander Aprelev <aam@google.com>
Commit-Queue: Alexander Aprelev <aam@google.com>
diff --git a/pkg/pkg.status b/pkg/pkg.status
index c8f7401..3077d69 100644
--- a/pkg/pkg.status
+++ b/pkg/pkg.status
@@ -227,6 +227,7 @@
 analysis_server/test/analysis/get_errors_test: Skip # runtime error, Issue 22180
 analysis_server/test/benchmarks_test: RuntimeError # Issue 32355
 analysis_server/test/integration/*: Skip # Issue 32356
+analysis_server/test/src/plugin/plugin_manager_test: Pass, Slow, RuntimeError # Issue 34231
 analyzer/test/generated/non_error_resolver_kernel_test: RuntimeError # Issue 30785
 analyzer/test/src/task/strong/checker_test: Pass, Slow
 analyzer/tool/task_dependency_graph/check_test: Slow, Pass