**Do you want to request a *feature* or report a *bug*?**

bug

**What is the current behavior?**

`toWarnDev` calls `jestDiff(a, b)` as `jestDiff(normalizedMessage, expectedMessages[0])` and `jestDiff([normalizedMessage], expectedMessages)`:
https://github.com/facebook/react/blob/94518b068bf52196abea0c83f4c9926bfe2065c6/scripts/jest/matchers/toWarnDev.js#L49-L57

[`jestDiff(a, b)`](https://github.com/facebook/jest/blob/457776b2889a9be1ce8a2c636a23417264a98d99/packages/jest-diff/src/index.js#L54) [calls](https://github.com/facebook/jest/blob/457776b2889a9be1ce8a2c636a23417264a98d99/packages/jest-diff/src/index.js#L93) [`diffStrings(a, b)`](https://github.com/facebook/jest/blob/457776b2889a9be1ce8a2c636a23417264a98d99/packages/jest-diff/src/diff_strings.js#L249-L251) where by default `a` is annotated as `'Expected'` (green), `b` as `'Received'` (red).

https://github.com/facebook/jest/blob/457776b2889a9be1ce8a2c636a23417264a98d99/packages/jest-diff/src/diff_strings.js#L86-L90
```js
const getAnnotation = (options: ?DiffOptions): string =>
  chalk.green('- ' + ((options && options.aAnnotation) || 'Expected')) +
  '\n' +
  chalk.red('+ ' + ((options && options.bAnnotation) || 'Received')) +
  '\n\n';
```

**What is the expected behavior?**

`toWarnDev` should pass the expected values into `a`, the actual (received) values into `b` of `jestDiff`:
```js
jestDiff(expectedMessages[0], normalizedMessage)
```
```js
jestDiff(expectedMessages, [normalizedMessage])
```
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'),
};