Check for cyclic type variable bounds.

sdk/lib/_internal/compiler/implementation/warnings.dart

 // Copyright (c) 2011, 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.
 
 part of dart2js;
 
 class MessageKind {
   final String template;
   const MessageKind(this.template);
 
@@ skipping 173 matching lines @@
   static const CANNOT_INSTANTIATE_TYPEDEF = const MessageKind(
       "cannot instantiate typedef '#{typedefName}'");
 
   static const CANNOT_INSTANTIATE_TYPE_VARIABLE = const MessageKind(
       "cannot instantiate type variable '#{typeVariableName}'");
 
   static const NO_DEFAULT_CLASS = const MessageKind(
       "no default class on enclosing interface '#{interfaceName}'");
 
   static const CYCLIC_TYPE_VARIABLE = const MessageKind(
-      "cyclic reference to type variable #{typeVariableName}");
+      "type variable #{typeVariableName} is a supertype of itself");
 
   static const CLASS_NAME_EXPECTED = const MessageKind(
       "class name expected");
 
   static const INTERFACE_TYPE_EXPECTED = const MessageKind(
       "interface type expected");
 
   static const CANNOT_EXTEND = const MessageKind(
       "#{type} cannot be extended");
 
@@ skipping 369 matching lines @@
 
 class CompileTimeConstantError extends Diagnostic {
   CompileTimeConstantError(MessageKind kind, [Map arguments = const {}])
     : super(kind, arguments);
 }
 
 class CompilationError extends Diagnostic {
   CompilationError(MessageKind kind, [Map arguments = const {}])
     : super(kind, arguments);
 }