Use an OWNERS file to restrict editing of tests/corelib to language team.

Change-Id: Ia42a8d9b7be44465e880685c790fe193721c2c04
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/360900
Auto-Submit: Paul Berry <paulberry@google.com>
Commit-Queue: Alexander Thomas <athom@google.com>
Reviewed-by: Alexander Thomas <athom@google.com>

tests/corelib/OWNERS

@@ -0,0 +1,9 @@
+# Parent grants access to everybody so ignore that.
+set noparent
+
+# But we do still want to permit the last resort global approvers
+# defined at top level.
+file:/OWNERS
+
+# Owned by language team members.
+file:/tools/OWNERS_LANGUAGE