Report errors on repeated interfaces.

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

Index: sdk/lib/_internal/compiler/implementation/warnings.dart
diff --git a/sdk/lib/_internal/compiler/implementation/warnings.dart b/sdk/lib/_internal/compiler/implementation/warnings.dart
index 1b63947e8db14e5a625f6a9e615b19c95e3a5766..78ed8274c83f58e78626775227630047f015bcbd 100644
--- a/sdk/lib/_internal/compiler/implementation/warnings.dart
+++ b/sdk/lib/_internal/compiler/implementation/warnings.dart
@@ -200,6 +200,12 @@ class MessageKind {
   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");