Implement field parameter for generative constructors.

frog/leg/lib/core.dart

Index: frog/leg/lib/core.dart
===================================================================
--- frog/leg/lib/core.dart	(revision 3537)
+++ frog/leg/lib/core.dart	(working copy)
@@ -336,7 +336,7 @@
 class ListIterator<T> /* implements Iterator<T> */ {
   int i;
   List<T> list;
-  ListIterator(List<T> list) : this.list = list, i = 0;
+  ListIterator(List<T> this.list) : i = 0;
   bool hasNext() => i < JS("int", @"$0.length", list);
   T next() {
     var value = JS("Object", @"$0[$1]", list, i);