<img width="953" alt="screen shot 2018-03-22 at 4 47 40 pm" src="https://user-images.githubusercontent.com/810438/37785026-037d0788-2df1-11e8-9cbd-5b9e2b2d10db.png">

Caused by this line:

https://github.com/facebook/react/blob/40fa6160534e2bc67b0c43f0b32dcdd1d214068b/scripts/jest/matchers/toWarnDev.js#L52

Calling `jestDiff` with two different strings returns `null`.
'use strict';
const jestDiff = require('jest-diff');
function normalizeCodeLocInfo(str) {
  return str && str.replace(/at .+?:\d+/g, 'at **');
}
const createMatcherFor = consoleMethod =>
  function matcher(callback, expectedMessages) {
    if (__DEV__) {
      
      if (typeof expectedMessages === 'string') {
        expectedMessages = [expectedMessages];
      } else if (!Array.isArray(expectedMessages)) {