<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`.
if (unexpectedWarnings.length > 0) {
          return {
            message: () => unexpectedWarnings[0].stack,
            pass: false,
          };
        }
        
        if (expectedMessages.length > 0) {
          return {
            message: () =>
              `Expected warning was not recorded:\n  ${this.utils.printReceived(
                expectedMessages[0]
              )}`,
            pass: false,
          };
        }
        return {pass: true};
      }
    } else {
      
      callback();
      return {pass: true};
    }
  };
module.exports = {
  toLowPriorityWarnDev: createMatcherFor('warn'),
  toWarnDev: createMatcherFor('error'),
};