Implement cycle-checking and code generation for redirecting constructors.

frog/leg/warnings.dart

Index: frog/leg/warnings.dart
diff --git a/frog/leg/warnings.dart b/frog/leg/warnings.dart
index 146ae7209c11c1c0877f22d1a528abd2838fdb39..d9a0b0baa962ace9fe038f528efea2095dccbde4 100644
--- a/frog/leg/warnings.dart
+++ b/frog/leg/warnings.dart
@@ -68,7 +68,11 @@ class MessageKind {
       "only call to 'this' or 'super' constructor allowed");
   static final INVALID_FOR_IN = const MessageKind(
       'Invalid for-in variable declaration.');
-  static final REDIRECTING_CTOR_HAS_INITIALIZER = const MessageKind(
+  static final REDIRECTING_CONSTRUCTOR_CYCLE = const MessageKind(
+     'cyclic constructor redirection');
+  static final REDIRECTING_CONSTRUCTOR_HAS_BODY = const MessageKind(
+     'redirecting constructor cannot have a body');
+  static final REDIRECTING_CONSTRUCTOR_HAS_INITIALIZER = const MessageKind(
       'redirecting constructor cannot have other initializers');
   static final SUPER_INITIALIZER_IN_OBJECT = const MessageKind(
       "'Object' cannot have a super initializer");