Add warning if is is set and tries to update

packages/react-dom-bindings/src/client/ReactDOMComponent.js

@@ -657,6 +657,21 @@ function setProp(
  );
  break;
  // Properties that should not be allowed on custom elements.
+ case 'is': {
+ if (__DEV__) {
+ if (prevValue != null) {
+ console.error(
+ 'Cannot update the "is" prop after it has been initialized.',
+ );
+ }
+ }
+ // TODO: We shouldn't actually set this attribute, because we've already
+ // passed it to createElement. We don't also need the attribute.
+ // However, our tests currently query for it so it's plausible someone
+ // else does too so it's break.
+ setValueForAttribute(domElement, 'is', value);
+ break;
+ }
  case 'innerText':
  case 'textContent':
  if (enableCustomElementPropertySupport) {