<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`.
throw Error(
          `toWarnDev() requires a parameter of type string or an array of strings ` +
            `but was given ${typeof expectedMessages}.`
        );
      }
      const unexpectedWarnings = [];
      
      
      
      
      let caughtError;
      const consoleSpy = message => {
        const normalizedMessage = normalizeCodeLocInfo(message);
        for (let index = 0; index < expectedMessages.length; index++) {
          const expectedMessage = expectedMessages[index];
          if (
            normalizedMessage === expectedMessage ||
            normalizedMessage.includes(expectedMessage)
          ) {
            expectedMessages.splice(index, 1);
            return;
          }
        }
        let errorMessage;
        if (expectedMessages.length === 0) {
          errorMessage =
            'Unexpected warning recorded: ' +
            this.utils.printReceived(normalizedMessage);
        } else if (expectedMessages.length === 1) {
          errorMessage =