Allow the list of type parameters in the default factory clause to be omitted.

runtime/vm/parser.cc

Index: runtime/vm/parser.cc
===================================================================
--- runtime/vm/parser.cc	(revision 3579)
+++ runtime/vm/parser.cc	(working copy)
@@ -2804,7 +2804,8 @@
         mismatch = true;
       }
     }
-    if (mismatch) {
+    // The list of type parameters in the default factory clause can be omitted.
+    if (mismatch && (num_type_params > 0)) {
       const String& interface_name = String::Handle(interface.Name());
       const String& factory_name = String::Handle(factory_class.Name());
       ErrorMsg(factory_pos,