[Fiber] Use a safer strategy to track the last precedence (facebook#2…

…8110)

Uses a safer strategy to track the last precedence to avoid the need to
consistently remember to preprend `'p'` to the precedence value

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

@@ -3481,10 +3481,20 @@ function onUnsuspend(this: SuspendedState) {
   }
 }
 
+// We use a value that is type distinct from precedence to track which one is last.
+// This ensures there is no collision with user defined precedences. Normally we would
+// just track this in module scope but since the precedences are tracked per HoistableRoot
+// we need to associate it to something other than a global scope hence why we try to
+// colocate it with the map of precedences in the first place
+const LAST_PRECEDENCE = null;
+
 // This is typecast to non-null because it will always be set before read.
 // it is important that this not be used except when the stack guarantees it exists.
 // Currentlyt his is only during insertSuspendedStylesheet.
-let precedencesByRoot: Map<HoistableRoot, Map<string, Instance>> = (null: any);
+let precedencesByRoot: Map<
+  HoistableRoot,
+  Map<string | typeof LAST_PRECEDENCE, Instance>,
+> = (null: any);
 
 function insertSuspendedStylesheets(
   state: SuspendedState,
@@ -3539,27 +3549,27 @@ function insertStylesheetIntoRoot(
         // and will be hoisted by the Fizz runtime imminently.
         node.getAttribute('media') !== 'not all'
       ) {
-        precedences.set('p' + node.dataset.precedence, node);
+        precedences.set(node.dataset.precedence, node);
         last = node;
       }
     }
     if (last) {
-      precedences.set('last', last);
+      precedences.set(LAST_PRECEDENCE, last);
     }
   } else {
-    last = precedences.get('last');
+    last = precedences.get(LAST_PRECEDENCE);
   }
 
   // We only call this after we have constructed an instance so we assume it here
   const instance: HTMLLinkElement = (resource.instance: any);
   // We will always have a precedence for stylesheet instances
   const precedence: string = (instance.getAttribute('data-precedence'): any);
 
-  const prior = precedences.get('p' + precedence) || last;
+  const prior = precedences.get(precedence) || last;
   if (prior === last) {
-    precedences.set('last', instance);
+    precedences.set(LAST_PRECEDENCE, instance);
   }
-  precedences.set('p' + precedence, instance);
+  precedences.set(precedence, instance);
 
   this.count++;
   const onComplete = onUnsuspend.bind(this);