Change new Date() to Date.now() to save on date allocations

Summary:
`new Date().getTime()` is equal to `Date.now()`.
`Date.now()` avoids an allocation, which can save some GC time.

This micro-optimization isn't worth it in most places, but since MessageQueue is one of the hottest pieces of JS in RN, it's worth it.

Reviewed By: javache

Differential Revision: D9972334

fbshipit-source-id: 05d78fd65304f0f27115d76b8b52db11a52c86a0

Libraries/BatchedBridge/MessageQueue.js

@@ -60,7 +60,7 @@ class MessageQueue {
     this._failureCallbacks = {};
     this._callID = 0;
     this._lastFlush = 0;
-    this._eventLoopStartTime = new Date().getTime();
+    this._eventLoopStartTime = Date.now();
     this._immediatesCallback = null;
 
     if (__DEV__) {
@@ -141,7 +141,7 @@ class MessageQueue {
   }
 
   getEventLoopRunningTime() {
-    return new Date().getTime() - this._eventLoopStartTime;
+    return Date.now() - this._eventLoopStartTime;
   }
 
   registerCallableModule(name: string, module: Object) {
@@ -243,7 +243,7 @@ class MessageQueue {
     }
     this._queue[PARAMS].push(params);
 
-    const now = new Date().getTime();
+    const now = Date.now();
     if (
       global.nativeFlushQueueImmediate &&
       now - this._lastFlush >= MIN_TIME_BETWEEN_FLUSHES_MS
@@ -323,7 +323,7 @@ class MessageQueue {
   }
 
   __callFunction(module: string, method: string, args: any[]): any {
-    this._lastFlush = new Date().getTime();
+    this._lastFlush = Date.now();
     this._eventLoopStartTime = this._lastFlush;
     if (__DEV__ || this.__spy) {
       Systrace.beginEvent(`${module}.${method}(${stringifySafe(args)})`);
@@ -352,7 +352,7 @@ class MessageQueue {
   }
 
   __invokeCallback(cbID: number, args: any[]) {
-    this._lastFlush = new Date().getTime();
+    this._lastFlush = Date.now();
     this._eventLoopStartTime = this._lastFlush;
 
     // The rightmost bit of cbID indicates fail (0) or success (1), the other bits are the callID shifted left.