Add compilation error on named parameters with private names

runtime/vm/parser.cc

Index: runtime/vm/parser.cc
===================================================================
--- runtime/vm/parser.cc	(revision 26668)
+++ runtime/vm/parser.cc	(working copy)
@@ -1410,6 +1410,11 @@
     params->has_field_initializer = true;
   }
 
+  if (params->has_optional_named_parameters &&
+      (parameter.name->CharAt(0) == '_')) {
+    ErrorMsg(parameter.name_pos, "named parameter must not be private");
+  }
+
   // Check for duplicate formal parameters.
   const intptr_t num_existing_parameters =
       params->num_fixed_parameters + params->num_optional_parameters;