function isNative(feat) {
    var S = window.Symbol;
    return !!(
      feat &&
      isFunction(S) &&
      isFunction(S.toString) &&
      S(feat).toString().indexOf(nativeCode) > -1
    );
  }
Check if a value (e.g. a method or constructor) is native code. Good for
detecting when a polyfill is used and when not.

@param {*} feat
@returns {Boolean}