jsx(): Treat __self and __source as normal props

These used to be reserved props because the classic React.createElement
runtime passed this data as props, whereas the jsxDEV() runtime passes
them as separate arguments.

This brings us incrementally closer to being able to pass the props
object directly through to React instead of cloning a subset into a
new object.

The React.createElement runtime is unaffected.

Author: acdlite

packages/react/src/__tests__/ReactElementValidator-test.internal.js

@@ -569,4 +569,26 @@ describe('ReactElementValidator', () => {
     React.createElement(Lazy);
     expect(didCall).toBe(false);
   });
+
+  it('__self and __source are treated as normal props', async () => {
+    // These used to be reserved props because the classic React.createElement
+    // runtime passed this data as props, whereas the jsxDEV() runtime passes
+    // them as separate arguments.
+    function Child({__self, __source}) {
+      return __self + __source;
+    }
+
+    const container = document.createElement('div');
+    const root = ReactDOMClient.createRoot(container);
+    // NOTE: The Babel transform treats the presence of these props as a syntax
+    // error but theoretically it doesn't have to. Using spread here to
+    // circumvent the syntax error and demonstrate that the runtime
+    // doesn't care.
+    const props = {
+      __self: 'Hello ',
+      __source: 'world!',
+    };
+    await act(() => root.render(<Child {...props} />));
+    expect(container.textContent).toBe('Hello world!');
+  });
 });

packages/react/src/jsx/ReactJSXElement.js

@@ -16,8 +16,6 @@ const ReactCurrentOwner = ReactSharedInternals.ReactCurrentOwner;
 const RESERVED_PROPS = {
   key: true,
   ref: true,
-  __self: true,
-  __source: true,
 };
 
 let specialPropKeyWarningShown;