anchor.diff:      return new SassIdentifier("$name(${arguments.join(', ')})");
     }
   
  -  /*=T*/ _runUserDefinedCallable/*<T>*/(CallableInvocation invocation,
  -      UserDefinedCallable callable, /*=T*/ run()) {
  +  Value _runUserDefinedCallable(CallableInvocation invocation,
  +      UserDefinedCallable callable, Value run()) {
  +    var pair = _evaluateArguments(invocation);
  +    var positional = pair.first;
  +    var named = pair.last;
  +
       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
  
positive.code:       for (var statement in mixin.declaration.children) {
          statement.accept(this);
        }
      });
    }
  
  
negative.code:             new SassList(rest, ListSeparator.comma));
        }
  
        return run();
      }));
    }
  