function toSource(func) {
  if (func != null) {
    try {
      return funcToString.call(func);
    } catch (e) {}
    try {
      return (func + '');
    } catch (e) {}
  }
  return '';
}
Converts `func` to its source code.

@private
@param {Function} func The function to convert.
@returns {string} Returns the source code.