Allow for-in to use a previously declared identifier as variable.

frog/leg/warnings.dart

Index: frog/leg/warnings.dart
diff --git a/frog/leg/warnings.dart b/frog/leg/warnings.dart
index 99553a7782c09e265a337bdf39ef49945f32f510..2c0d9a8250bf50f2150b642a8708391f82a936e3 100644
--- a/frog/leg/warnings.dart
+++ b/frog/leg/warnings.dart
@@ -58,6 +58,8 @@ class MessageKind {
       'cannot initialize static field #{1}');
   static final NOT_A_FIELD = const MessageKind(
       '#{1} is not a field');
+  static final INVALID_FOR_IN = const MessageKind(
+      'Invalid for-in variable declaration.');
 
   toString() => template;
 }