Bug: Event handlers on custom elements work on the client but not on the server

<!-- Please provide a clear and concise description of what the bug is. Include screenshots if needed. Please test using the latest version of the relevant React packages to make sure your issue has not already been fixed. --> React version: 16.13.1 ## Steps To Reproduce ```js ReactDOMServer.renderToString(<div-x onClick={() => console.log('clicked')} />) ``` vs ```js ReactDOM.render(<div-x onClick={() => { console.log('clicked'); }} />, root) ``` <!-- Your bug will get fixed much faster if we can run your code and it doesn't have dependencies other than React. Issues without reproduction steps or code examples may be immediately closed as not actionable. --> Link to code example: https://jsfiddle.net/hsug65x0/2/ <!-- Please provide a CodeSandbox (https://codesandbox.io/s/new), a link to a repository on GitHub, or provide a minimal code example that reproduces the problem. You may provide a screenshot of the application if you think it is relevant to your bug report. Here are some tips for providing a minimal example: https://stackoverflow.com/help/mcve. --> ## The current behavior Custom element event handlers only work client side. In SSR the code of the event handler becomes an attribute value. ## The expected behavior Custom element event handlers work in SSR.