anchor.diff:      bool isFunctionChecked = false;
   
       void processCheckedType(DartType t) {
  +      t = t.withoutNullability;
         if (t is FunctionType) {
           checkedFunctionTypes.add(t);
         } else if (t is InterfaceType) {
  
positive.code: 
                assert(substitution != null);
                for (DartType argument in substitution.arguments) {
                  if (argument is InterfaceType) {
                    computeChecks(argument.element);
                  }
  
negative.code: 
                  assert(substitution != null);
                  for (DartType argument in substitution.arguments) {
                    argument = argument.withoutNullability;
                    if (argument is InterfaceType) {
                      computeChecks(argument.element);
                    }
  