Implement new optional parameters syntax in the vm (issue 4290).

runtime/vm/dart_api_impl.cc

Index: runtime/vm/dart_api_impl.cc
===================================================================
--- runtime/vm/dart_api_impl.cc	(revision 11998)
+++ runtime/vm/dart_api_impl.cc	(working copy)
@@ -2839,7 +2839,9 @@
   // provide them in the argument lists they are handed.
   *fixed_param_count = (func.num_fixed_parameters() -
                         func.NumberOfImplicitParameters());
-  *opt_param_count = func.num_optional_parameters();
+  // TODO(regis): Separately report named and positional optional param counts.
+  *opt_param_count = (func.num_optional_positional_parameters() +
+                      func.num_optional_named_parameters());
 
   ASSERT(*fixed_param_count >= 0);
   ASSERT(*opt_param_count >= 0);