toString() {
    var block
      , node
      , buf = []
      , location
      , len = this.length;

    while (len--) {
      block = this[len].block;
      if (node = block.node) {
        location = '(' + node.filename + ':' + (node.lineno + 1) + ':' + node.column + ')';
        switch (node.nodeName) {
          case 'function':
            buf.push('    at ' + node.name + '() ' + location);
            break;
          case 'group':
            buf.push('    at "' + node.nodes[0].val + '" ' + location);
            break;
        }
      }
    }

    return buf.join('\n');
  }
Return stack string formatted as:

  at <context> (<filename>:<lineno>:<column>)

@return {String}
@api private