Replace pendingWorkPriority with expiration times

Instead of a priority, a fiber has an expiration time that represents
a point in the future by which it should render.

Pending updates still have priorities so that they can be coalesced.

We use a host config method to read the current time. This commit
implements everything except that method, which currently returns a
constant value. So this just proves that expiration times work the same
as priorities when time is frozen. Subsequent commits will show the
effect of advancing time.

src/renderers/dom/fiber/ReactDOMFiberEntry.js

@@ -164,20 +164,20 @@ function shouldAutoFocusHostComponent(type: string, props: Props): boolean {
 }
 
 // TODO: Better polyfill
-let now;
-if (
-  typeof window !== 'undefined' &&
-  window.performance &&
-  typeof window.performance.now === 'function'
-) {
-  now = function() {
-    return performance.now();
-  };
-} else {
-  now = function() {
-    return Date.now();
-  };
-}
+// let now;
+// if (
+//   typeof window !== 'undefined' &&
+//   window.performance &&
+//   typeof window.performance.now === 'function'
+// ) {
+//   now = function() {
+//     return performance.now();
+//   };
+// } else {
+//   now = function() {
+//     return Date.now();
+//   };
+// }
 
 var DOMRenderer = ReactFiberReconciler({
   getRootHostContext(rootContainerInstance: Container): HostContext {
@@ -447,7 +447,10 @@ var DOMRenderer = ReactFiberReconciler({
     }
   },
 
-  now: now,
+  now() {
+    // TODO: Use performance.now to enable expiration
+    return 0;
+  },
 
   canHydrateInstance(
     instance: Instance | TextInstance,

src/renderers/noop/ReactNoopEntry.js

@@ -412,15 +412,15 @@ var ReactNoop = {
         '  '.repeat(depth + 1) + '~',
         firstUpdate && firstUpdate.partialState,
         firstUpdate.callback ? 'with callback' : '',
-        '[' + firstUpdate.priorityLevel + ']',
+        '[' + firstUpdate.expirationTime + ']',
       );
       var next;
       while ((next = firstUpdate.next)) {
         log(
           '  '.repeat(depth + 1) + '~',
           next.partialState,
           next.callback ? 'with callback' : '',
-          '[' + firstUpdate.priorityLevel + ']',
+          '[' + firstUpdate.expirationTime + ']',
         );
       }
     }
@@ -430,7 +430,7 @@ var ReactNoop = {
         '  '.repeat(depth) +
           '- ' +
           (fiber.type ? fiber.type.name || fiber.type : '[root]'),
-        '[' + fiber.pendingWorkPriority + (fiber.pendingProps ? '*' : '') + ']',
+        '[' + fiber.expirationTime + (fiber.pendingProps ? '*' : '') + ']',
       );
       if (fiber.updateQueue) {
         logUpdateQueue(fiber.updateQueue, depth);