Report compile-time errors on cyclic hierarchies.

pkg/front_end/tool/_fasta/generate_messages.dart

 // Copyright (c) 2017, 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.
 
 import 'dart:io';
 
 import 'dart:isolate';
 
-import 'dart:convert';
-
 import 'package:yaml/yaml.dart' show loadYaml;
 
 import 'package:front_end/src/fasta/parser/error_kind.dart' show ErrorKind;
 
 main(List<String> arguments) async {
   var port = new ReceivePort();
   Uri messagesFile = Platform.script.resolve("../../messages.yaml");
   Map yaml = loadYaml(await new File.fromUri(messagesFile).readAsStringSync());
   Set<String> names =
       new Set<String>.from(yaml.keys.map((String s) => "ErrorKind.$s"));
@@ skipping 91 matching lines @@
   ${interpolate('message', template)}
   ${interpolate('tip', tip)}
     'code': ErrorKind.$name,
     'arguments': {
       ${arguments.join('\n      ')}
     },
   };
 }
 """;
 }