Remove exported constants from RCTTimers to allow lazy initialization

Summary:
As per javache comments in #8734.

Also removes now useless feature detection check.

**Test plan**
Tested that rIC still works in UIExplorer example.
Closes https://github.com/facebook/react-native/pull/8795

Differential Revision: D3572566

Pulled By: javache

fbshipit-source-id: 261d13d8b03898313f8b4184d634c70f81a61b62

Libraries/JavaScriptAppEngine/System/JSTimers/JSTimers.js

@@ -101,11 +101,6 @@ var JSTimers = {
    * with time remaining in frame.
    */
   requestIdleCallback: function(func) {
-    if (!RCTTiming.setSendIdleEvents) {
-      console.warn('requestIdleCallback is not currently supported on this platform');
-      return requestAnimationFrame(func);
-    }
-
     if (JSTimersExecution.requestIdleCallbacks.length === 0) {
       RCTTiming.setSendIdleEvents(true);
     }

Libraries/JavaScriptAppEngine/System/JSTimers/JSTimersExecution.js

@@ -17,6 +17,11 @@ const keyMirror = require('fbjs/lib/keyMirror');
 const performanceNow = require('fbjs/lib/performanceNow');
 const warning = require('fbjs/lib/warning');
 
+// These timing contants should be kept in sync with the ones in native ios and
+// android `RCTTiming` module.
+const FRAME_DURATION = 1000 / 60;
+const IDLE_CALLBACK_FRAME_DEADLINE = 1;
+
 let hasEmittedTimeDriftWarning = false;
 
 /**
@@ -81,13 +86,12 @@ const JSTimersExecution = {
         const currentTime = performanceNow();
         callback(currentTime);
       } else if (type === JSTimersExecution.Type.requestIdleCallback) {
-        const { Timing } = require('NativeModules');
         callback({
           timeRemaining: function() {
             // TODO: Optimisation: allow running for longer than one frame if
             // there are no pending JS calls on the bridge from native. This
             // would require a way to check the bridge queue synchronously.
-            return Math.max(0, Timing.frameDuration - (performanceNow() - frameTime));
+            return Math.max(0, FRAME_DURATION - (performanceNow() - frameTime));
           },
         });
       } else {
@@ -134,7 +138,7 @@ const JSTimersExecution = {
   callIdleCallbacks: function(frameTime) {
     const { Timing } = require('NativeModules');
 
-    if (Timing.frameDuration - (performanceNow() - frameTime) < Timing.idleCallbackFrameDeadline) {
+    if (FRAME_DURATION - (performanceNow() - frameTime) < IDLE_CALLBACK_FRAME_DEADLINE) {
       return;
     }
 

React/Modules/RCTTiming.m

@@ -16,6 +16,8 @@
 #import "RCTUtils.h"
 
 static const NSTimeInterval kMinimumSleepInterval = 1;
+
+// These timing contants should be kept in sync with the ones in `JSTimersExecution.js`.
 // The duration of a frame. This assumes that we want to run at 60 fps.
 static const NSTimeInterval kFrameDuration = 1.0 / 60.0;
 // The minimum time left in a frame to trigger the idle callback.
@@ -138,14 +140,6 @@ RCT_EXPORT_MODULE()
   return RCTJSThread;
 }
 
-- (NSDictionary *)constantsToExport
-{
-  return @{
-    @"frameDuration": @(kFrameDuration * 1000),
-    @"idleCallbackFrameDeadline": @(kIdleCallbackFrameDeadline * 1000),
-  };
-}
-
 - (void)invalidate
 {
   [self stopTimers];

ReactAndroid/src/main/java/com/facebook/react/modules/core/Timing.java

@@ -45,6 +45,7 @@ import javax.annotation.Nullable;
 public final class Timing extends ReactContextBaseJavaModule implements LifecycleEventListener,
   OnExecutorUnregisteredListener {
 
+  // These timing contants should be kept in sync with the ones in `JSTimersExecution.js`.
   // The minimum time in milliseconds left in the frame to call idle callbacks.
   private static final float IDLE_CALLBACK_FRAME_DEADLINE_MS = 1.f;
   // The total duration of a frame in milliseconds, this assumes that devices run at 60 fps.
@@ -309,13 +310,6 @@ public final class Timing extends ReactContextBaseJavaModule implements Lifecycl
     return true;
   }
 
-  @Override
-  public Map<String, Object> getConstants() {
-    return MapBuilder.<String, Object>of(
-        "frameDuration", FRAME_DURATION_MS,
-        "idleCallbackFrameDeadline", IDLE_CALLBACK_FRAME_DEADLINE_MS);
-  }
-
   @Override
   public void onExecutorDestroyed(ExecutorToken executorToken) {
     synchronized (mTimerGuard) {