function isEqualAst(a, b) {
  return isEqualWith((aValue, bValue, key) => metaDataFields[key], a, b);
}
True when two AST nodes are structurally equal.
When comparing objects it ignores the meta-data fields for
comments and source-code position.
@param  {Object}  a
@param  {Object}  b
@return {Boolean}