Report compile-time errors on cyclic hierarchies.

pkg/front_end/lib/src/fasta/builder/class_builder.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library fasta.class_builder;
 
 import '../errors.dart' show internalError;
 
 import 'builder.dart'
     show
@@ skipping 169 matching lines @@
           if (substitutionMap != null) {
             argument = argument.subst(substitutionMap);
           }
           directSubstitutionMap[variables[i]] = argument;
         }
         substitutionMap = directSubstitutionMap;
       }
     }
     return substitutionMap;
   }
+
+  void addCompileTimeError(int charOffset, String message) {
+    library.addCompileTimeError(charOffset, message, fileUri: fileUri);
+  }
+
+  void addWarning(int charOffset, String message) {
+    library.addWarning(charOffset, message, fileUri: fileUri);
+  }
+
+  void addNit(int charOffset, String message) {
+    library.addNit(charOffset, message, fileUri: fileUri);

[Paul Berry, 2017/03/22 16:22:30]

Not a blocking issue for this CL, but when we next meet I'd like to understand
what a "nit" is.  We've been hearing reports for quite some time that users are
confused by the subtle distinctions analyzer makes among error, warning, hint,
and lint, and we've been wanting to reduce the number of categories.  Adding a
"nit" category seems like it goes against this goal.

[ahe, 2017/03/22 17:49:49]

I agree. I use "nit" to report warnings that aren't warnings. Perhaps I should
silence them unless using verbose mode.

[ahe, 2017/03/23 11:35:35]

I did that in CL 2770683005.

+  }
 }