[docs] Enable StrictMode

[oliviertassinari]

I'm not eager to see other occurrences of mui/mui-x#177 going forward. I couldn't notice any errors in development. I could test that the switch work with:

diff --git a/docs/src/pages/components/alert/BasicAlerts.tsx b/docs/src/pages/components/alert/BasicAlerts.tsx
index b0525db44c..bd9b09e0d8 100644
--- a/docs/src/pages/components/alert/BasicAlerts.tsx
+++ b/docs/src/pages/components/alert/BasicAlerts.tsx
@@ -1,4 +1,5 @@
 import * as React from 'react';
+import ReactDOM from 'react-dom';
 import { makeStyles, Theme, createStyles } from '@material-ui/core/styles';
 import Alert from '@material-ui/core/Alert';

@@ -13,12 +14,23 @@ const useStyles = makeStyles((theme: Theme) =>
   }),
 );

+class Foo extends React.Component {
+  render() {
+    return <div>hello</div>
+  }
+}
+
 export default function BasicAlerts() {
   const classes = useStyles();
+  const ref = React.useRef();
+
+  React.useEffect(() => {
+    ReactDOM.findDOMNode(ref.current);
+  });

   return (
     <div className={classes.root}>
-      <Alert severity="error">This is an error alert — check it out!</Alert>
+      <Foo ref={ref} severity="error">This is an error alert — check it out!</Foo>
       <Alert severity="warning">This is a warning alert — check it out!</Alert>
       <Alert severity="info">This is an info alert — check it out!</Alert>
       <Alert severity="success">This is a success alert — check it out!</Alert>

[mui-pr-bot]

Details of bundle changes

Generated by 🚫 dangerJS against ffa8b4f

[eps1lon]

As long as we use @material-ui/styles we can't use StrictMode.

Try navigating pages with yarn docs:dev. Styles should be broken.

If you find a bug then you should write an automated test. Manual testing does not scale with our team size.

[oliviertassinari]

I couldn't spot any bugs navigating the pages. I believe the bugs we have are related to the usage of dynamic props #18018, which we barely use and are progressively moving to emotion. I also recall one around global class name styles that aren't removed, however, the global styles we have are meant to be generic (they don't cause any visual issue I could spot).

[eps1lon]

I believe the bugs we have are related to the usage of dynamic props #18018, which we barely use and are progressively moving to emotion

But we still do and as long as these remains we can't switch. It would make local development just annoying.

[oliviertassinari]

Ok, for later then. I will enable it on material-ui-x to see how that goes.

[oliviertassinari]

Well no, I can't do that on material-ui-x, it's broken with the v4 documentation 😆, can only on next.