Avoid mangling the name of native methods.

frog/type.dart

Index: frog/type.dart
diff --git a/frog/type.dart b/frog/type.dart
index 4dafd284c6a17a8306a0dd2e026b74a0acf31116..c22cc57887212a8781d3766c80a3e76f118bb13f 100644
--- a/frog/type.dart
+++ b/frog/type.dart
@@ -915,11 +915,9 @@ class DefinedType extends Type {
       if (definition.values != null) {
         value = definition.values[i];
       }
-      var field = new FieldMember(name, this, definition, value);
+      var field =
+          new FieldMember(name, this, definition, value, isNative: isNative);
       members[name] = field;
-      if (isNative) {
-        field.isNative = true;
-      }
     }
   }