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;
}
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.warn()`.

@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