named optional parameters must not be private

runtime/vm/parser.cc

Index: runtime/vm/parser.cc
===================================================================
--- runtime/vm/parser.cc	(revision 5438)
+++ runtime/vm/parser.cc	(working copy)
@@ -893,6 +893,11 @@
   // At this point, we must see an identifier for the parameter name.
   parameter.name_pos = token_index_;
   parameter.name = ExpectIdentifier("parameter name expected");
+  if (params->has_named_optional_parameters &&
+      (parameter.name->CharAt(0) == '_')) {
+    ErrorMsg(parameter.name_pos,
+             "named optional parameters may not start with '_'\n");
+  }
 
   if (parameter.is_field_initializer) {
     params->has_field_initializer = true;