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

runtime/vm/class_finalizer.cc

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/class_finalizer.h"
 
 #include "vm/flags.h"
 #include "vm/heap.h"
 #include "vm/isolate.h"
 #include "vm/longjump.h"
@@ skipping 387 matching lines @@
   for (intptr_t i = 0; !mismatch && (i < num_type_params); i++) {
     expected_type_name ^= expected_type_names.At(i);
     actual_type_name ^= actual_type_names.At(i);
     expected_type_extends = expected_extends_array.TypeAt(i);
     actual_type_extends = actual_extends_array.TypeAt(i);
     if (!expected_type_name.Equals(actual_type_name) ||
         !expected_type_extends.Equals(actual_type_extends)) {
       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());
     const Script& script = Script::Handle(interface.script());
     ReportError(script, unresolved_factory_class.token_index(),
                 "mismatch in number, names, or bounds of type parameters "
                 "between default clause of interface '%s' and actual factory "
                 "class '%s'.\n",
                 interface_name.ToCString(),
                 factory_name.ToCString());
   }
@@ skipping 824 matching lines @@
   va_end(args);
   if (FLAG_warning_as_error) {
     Isolate::Current()->long_jump_base()->Jump(1, error);
     UNREACHABLE();
   } else {
     OS::Print("%s", error.ToErrorCString());
   }
 }
 
 }  // namespace dart