Summary:
Accessing the `responseText` property when `responseType` is not `''` or `'text'` should throw. Also, the property is read-only.
**Test Plan:** UIExplorer example, unit tests
Closes https://github.com/facebook/react-native/pull/7284
Differential Revision: D3366893
fbshipit-source-id: a4cf5ebabcd1e03d6e2dc9d51230982922746c11