[polyfill] Add sentinel to array

To make the polyfill work well with devtools, we add this sentinel.

Devtools will look for this sentinel before adding the Forget badge.

ghstack-source-id: d246040f67da48fa46f818753c8bf26dff56f390
Pull Request resolved: https://github.com/facebook/react-forget/pull/2847

compiler/packages/react-forget-runtime/src/index.ts

@@ -28,6 +28,10 @@ export function unstable_useMemoCache(size: number) {
     for (let ii = 0; ii < size; ii++) {
       $[ii] = $empty;
     }
+    // This symbol is added to tell the react devtools that this array is from
+    // useMemoCache.
+    // @ts-ignore
+    $[$empty] = true;
     return $;
   })[0];
 }