fix(act): add a polyfill

Thanks @threepointone!

Author: Kent C. Dodds

src/__tests__/no-act.js

@@ -6,5 +6,4 @@ test('act works even when there is no act from test utils', () => {
   const callback = jest.fn()
   act(callback)
   expect(callback).toHaveBeenCalledTimes(1)
-  expect(callback).toHaveBeenCalledWith(/* nothing */)
 })

src/act-compat.js

@@ -1,9 +1,16 @@
+import ReactDOM from 'react-dom'
 import {act as reactAct} from 'react-dom/test-utils'
 
 // act is supported react-dom@16.8.0
-// and is only needed for versions higher than that
-// so we do nothing for versions that don't support act.
-const act = reactAct || (cb => cb())
+// so for versions that don't have act from test utils
+// we do this little polyfill. No warnings, but it's
+// better than nothing.
+function actPolyfill(cb) {
+  ReactDOM.unstable_batchedUpdates(cb)
+  ReactDOM.render(null, document.createElement('div'))
+}
+
+const act = reactAct || actPolyfill
 
 function rtlAct(...args) {
   return act(...args)