Skip the enumerating mirror tests on debug Dartium.

tests/lib/lib.status

Index: tests/lib/lib.status
diff --git a/tests/lib/lib.status b/tests/lib/lib.status
index d07b8816b97bd4583dacfaff5fe65288316596f2..16dfa088cf3522ce9f5eddc4bd5eaef8a74db508 100644
--- a/tests/lib/lib.status
+++ b/tests/lib/lib.status
@@ -255,8 +255,8 @@ async/timer_test: Fail, Pass # Issue 15487
 async/multiple_timer_test: Fail, Pass # Issue 15487
 async/stream_periodic3_test: Fail, Pass # Issue 15487
 async/timer_isolate_test: Fail, Pass # Issue 15487. Issue 13921: spawnFunction is not allowed on Dartium's DOM thread.
-mirrors/immutable_collections_test: Pass, Slow # Dartium debug uses -O0
-mirrors/mirrors_reader_test: Pass, Slow # Dartium debug uses -O0
+mirrors/immutable_collections_test: Skip # Dartium debug uses -O0, so this will just timeout and waste bot cycles.
+mirrors/mirrors_reader_test: Skip # Dartium debug uses -O0, so this will just timeout and waste bot cycles.
 mirrors/library_uri_io_test: Skip # Not intended for drt as it uses dart:io.
 mirrors/local_isolate_test: Skip # http://dartbug.com/12188
 mirrors/deferred*: Skip # Lazy deferred loading not yet implemented. Issue 19449. Crashes drt in checked mode.