Follow factory redirections when checking case expressions.

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 c71f995cdeb3c0d859f1be8bb30dff5b501693aa..8ddc8d9c7cc9604cfe79eb3d74d72dd0ec038587 100644
--- a/sdk/lib/_internal/compiler/implementation/warnings.dart
+++ b/sdk/lib/_internal/compiler/implementation/warnings.dart
@@ -540,6 +540,9 @@ main() => new C<String>();
   static const MessageKind SWITCH_CASE_TYPES_NOT_EQUAL_CASE = const MessageKind(
       "'case' expression of type '#{type}'.");
 
+  static const MessageKind SWITCH_CASE_FORBIDDEN = const MessageKind(
+      "'case' expression may not be of type '#{type}'.");
+
   static const MessageKind SWITCH_CASE_VALUE_OVERRIDES_EQUALS =
       const MessageKind(
           "'case' expression type '#{type}' overrides 'operator =='.");