Report compile-time errors on cyclic hierarchies.

pkg/front_end/lib/src/fasta/source/source_loader.dart

Index: pkg/front_end/lib/src/fasta/source/source_loader.dart
diff --git a/pkg/front_end/lib/src/fasta/source/source_loader.dart b/pkg/front_end/lib/src/fasta/source/source_loader.dart
index 6227376d3513782d1e50ae6ab95f66a1f85cef83..d3e8b7f9ac06440cc07dde160bbcafdbc6c6712f 100644
--- a/pkg/front_end/lib/src/fasta/source/source_loader.dart
+++ b/pkg/front_end/lib/src/fasta/source/source_loader.dart
@@ -26,8 +26,6 @@ import '../export.dart' show Export;
 
 import '../loader.dart' show Loader;
 
-import '../messages.dart' show warning;
-
 import '../parser/class_member_parser.dart' show ClassMemberParser;
 
 import '../scanner.dart' show ErrorToken, ScannerResult, Token, scan;
@@ -336,8 +334,7 @@ class SourceLoader<L> extends Loader<L> {
             reported.add(cls);
           }
         }
-        warning(
-            cls.fileUri,
+        cls.addCompileTimeError(
             cls.charOffset,
             "'${cls.name}' is a supertype of "
             "itself via '${involved.map((c) => c.name).join(' ')}'.");