Make flow check async

Libraries/JavaScriptAppEngine/Initialization/InitializeJavaScriptAppEngine.js

@@ -59,6 +59,13 @@ function setUpRedBoxConsoleErrorHandler() {
   }
 }
 
+function setupFlowChecker() {
+  if (__DEV__) {
+    var checkFlowAtRuntime = require('checkFlowAtRuntime');
+    checkFlowAtRuntime();
+  }
+}
+
 /**
  * Sets up a set of window environment wrappers that ensure that the
  * BatchedBridge is flushed after each tick. In both the case of the
@@ -143,3 +150,4 @@ setUpGeolocation();
 setUpWebSockets();
 setupProfile();
 setUpProcessEnv();
+setupFlowChecker();