Implement field parameter for generative constructors.

frog/leg/warnings.dart

Index: frog/leg/warnings.dart
===================================================================
--- frog/leg/warnings.dart	(revision 3537)
+++ frog/leg/warnings.dart	(working copy)
@@ -76,9 +76,15 @@
       'no matching constructor found');
   static final NO_CONSTRUCTOR = const MessageKind(
       '#{1} is a #{2}, not a constructor');
+  static final FIELD_PARAMETER_NOT_ALLOWED = const MessageKind(
+      'A field parameter is only allowed in generative constructors');
+  static final INVALID_FIELD_PARAMETER = const MessageKind(
+      "A field parameter must be preceded by 'this'");
+  static final NOT_INSTANCE_FIELD = const MessageKind(
+      '#{1} is not an instance field');
 
   static final NOT_A_COMPILE_TIME_CONSTANT = const MessageKind(
-      '#{1} cannot be used as compile-time constant.');
+      '#{1} cannot be used as compile-time constant');
 
   toString() => template;
 }