anchor.diff:      }
   
       var result = _addExceptionSpan(invocation.span, () => callback(positional));
  +    _callableSpan = oldCallableSpan;
   
       if (argumentList == null) return result;
       if (named.isEmpty) return result;
  
positive.code:   /// The human-readable name of the current stack frame.
    var _member = "root stylesheet";
  
    /// The resolved URLs for each [ImportRule] that's been seen so far.
    ///
    /// This is cached in case the same file is imported multiple times, and thus
  
negative.code:       var args = arguments[1] as SassArgumentList;
  
        var expression = new FunctionExpression(
            new Interpolation([name.text], _callableSpan),
            new ArgumentInvocation([], {}, _callableSpan,
                rest: new ValueExpression(args)));
        return expression.accept(this);
      });
  