code: function deprecate (fn, msg) {
    if (config('noDeprecation')) {
      return fn;
    }
  
    var warned = false;
    function deprecated() {
      if (!warned) {
        if (config('throwDeprecation')) {
          throw new Error(msg);
        } else if (config('traceDeprecation')) {
          console.trace(msg);
        } else {
          console.warn(msg);
        }
        warned = true;
      }
      return fn.apply(this, arguments);
    }
  
    return deprecated;
  }
docs: Mark that a method should not be used.
  Returns a modified function which warns once by default.
  
  If `localStorage.noDeprecation = true` is set, then it is a no-op.
  
  If `localStorage.throwDeprecation = true` is set, then deprecated functions
  will throw an Error when invoked.
  
  If `localStorage.traceDeprecation = true` is set, then deprecated functions
  will invoke `console.trace()` instead of `console.error()`.
  
  @param {Function} fn - the function to deprecate
  @param {String} msg - the string to print to the console when `fn` is invoked
  @returns {Function} a new "deprecated" version of `fn`
  @api public