anchor.diff:        for (var statement in mixin.declaration.children) {
           statement.accept(this);
         }
  +      return null;
       });
     }
   
  
positive.code:     return new SassIdentifier("$name(${arguments.join(', ')})");
    }
  
    /*=T*/ _runUserDefinedCallable/*<T>*/(CallableInvocation invocation,
        UserDefinedCallable callable, /*=T*/ run()) {
      return _withEnvironment(callable.environment, () => _environment.scope(() {
        var pair = _evaluateArguments(invocation);
        var positional = pair.first;
        var named = pair.last;
  
        _verifyArguments(positional, named, callable.arguments, invocation.span);
  
        // TODO: if we get here and there are no rest params involved, mark the
  
negative.code:             new SassList(rest, ListSeparator.comma));
        }
  
        return run();
      }));
    }
  