Allow binary operators on compile-time-constants.

frog/leg/warnings.dart

Index: frog/leg/warnings.dart
diff --git a/frog/leg/warnings.dart b/frog/leg/warnings.dart
index 01cd9d0b73521f8f6df95fa4fc095b9777f9c841..55b38322b70f98a4cb8ac1b8342ec00d0c5842ea 100644
--- a/frog/leg/warnings.dart
+++ b/frog/leg/warnings.dart
@@ -108,7 +108,7 @@ class MessageKind {
       'target of continue is not a loop or switch case');
 
   static final NOT_A_COMPILE_TIME_CONSTANT = const MessageKind(
-      '#{1} cannot be used as compile-time constant');
+      'not a compile-time constant');
 
   static final NO_SUCH_LIBRARY_MEMBER = const MessageKind(
       '#{1} has no member named #{2}');