src/act-compat.js
function actPolyfill(cb) {
  ReactDOM.unstable_batchedUpdates(cb)
  ReactDOM.render(<div />, document.createElement('div'))
}
function actPolyfill(cb) {
  ReactDOM.unstable_batchedUpdates(cb)
  ReactDOM.render(<div />, document.createElement('div'))
}
function actPolyfill ( cb ) { ReactDOM . unstable_batchedUpdates ( cb ) ReactDOM . render ( < div / > , document . createElement ( 'div' ) ) }
act is supported react-dom@16.8.0 so for versions that don't have act from test utils we do this little polyfill. No warnings, but it's better than nothing.
act is supported react - dom