Add mirrors to whitelist again. It was removed in a previous cl, because it was

tests/compiler/dart2js/analyze_test_test.dart

Index: tests/compiler/dart2js/analyze_test_test.dart
diff --git a/tests/compiler/dart2js/analyze_test_test.dart b/tests/compiler/dart2js/analyze_test_test.dart
index bfe07acc27aca63c0b0fb0d631b57bc75031969d..f2565ac81e9dc46c30c59ffc981602d957ebfc8d 100644
--- a/tests/compiler/dart2js/analyze_test_test.dart
+++ b/tests/compiler/dart2js/analyze_test_test.dart
@@ -29,6 +29,7 @@ import 'memory_compiler.dart';
 const Map<String, List/*<String|MessageKind>*/> WHITE_LIST = const {
   // Several tests import mirrors; any of these might trigger the warning.
   ".dart": const [
+      MessageKind.IMPORT_EXPERIMENTAL_MIRRORS,
   ],
   "/test/src/util/": const [
       "Library 'package:async/async.dart' doesn't export a "