Report errors on repeated interfaces.

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 182 matching lines @@
 
   static const INTERFACE_TYPE_EXPECTED = const MessageKind(
       "interface type expected");
 
   static const CANNOT_EXTEND = const MessageKind(
       "#{1} cannot be extended");
 
   static const CANNOT_IMPLEMENT = const MessageKind(
       "#{1} cannot be implemented");
 
+  static const DUPLICATE_EXTENDS_IMPLEMENTS = const MessageKind(
+      "Error: #{1} can not be both extended and implemented.");
+
+  static const DUPLICATE_IMPLEMENTS = const MessageKind(
+      "Error: #{1} must not occur more than once in the implements clause.");
+
   static const ILLEGAL_SUPER_SEND = const MessageKind(
       "#{1} cannot be called on super");
 
   static const ADDITIONAL_TYPE_ARGUMENT = const MessageKind(
       "additional type argument");
 
   static const MISSING_TYPE_ARGUMENT = const MessageKind(
       "missing type argument");
 
   // TODO(johnniwinther): Use ADDITIONAL_TYPE_ARGUMENT or MISSING_TYPE_ARGUMENT
@@ skipping 247 matching lines @@
 
 class CompileTimeConstantError extends Diagnostic {
   CompileTimeConstantError(MessageKind kind, List arguments)
     : super(kind, arguments);
 }
 
 class CompilationError extends Diagnostic {
   CompilationError(MessageKind kind, List arguments)
     : super(kind, arguments);
 }