fixes #130, accept anything with `iterator` in for loops

lib/src/utils.dart

Index: lib/src/utils.dart
diff --git a/lib/src/utils.dart b/lib/src/utils.dart
index 1837b684905de08f9bc3d8d553f417043b11477d..12a5081f1c85e761625f4464b5c1d45e26560852 100644
--- a/lib/src/utils.dart
+++ b/lib/src/utils.dart
@@ -424,3 +424,16 @@ Map<String, DartType> getObjectMemberMap(TypeProvider typeProvider) {
   }
   return map;
 }
+
+/// Searches all supertype, in order of most derived members, to see if any
+/// [match] a condition. If so, returns the first match, otherwise returns null.
+InterfaceType findSupertype(InterfaceType type, bool match(InterfaceType t)) {
+  for (var m in type.mixins.reversed) {
+    if (match(m)) return m;
+  }
+  var s = type.superclass;
+  if (s == null) return null;
+
+  if (match(s)) return type;
+  return findSupertype(s, match);
+}