static isBefore(a, b) {
    a = a.loc.start;
    b = b.loc.start;
    return a.line < b.line || (a.line === b.line && a.column < b.column);
  }
Returns whether or not the first node appears before the second, by
comparing both their starting lines and columns.

@param   {object} a
@param   {object} b
@returns {bool}