[gnome-shell] ScreenShield: fix setting each arrow's opacity on _animateArrows()

[Rui Matos <rtcm src gnome org>]

commit e02467eada79528686df1152faf87858d3e9b3b6
Author: Rui Matos <tiagomatos gmail com>
Date:   Wed Jun 22 21:10:07 2016 +0200

    ScreenShield: fix setting each arrow's opacity on _animateArrows()
    
    https://bugzilla.gnome.org/show_bug.cgi?id=767954

 js/ui/screenShield.js |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/js/ui/screenShield.js b/js/ui/screenShield.js
index 18f5124..982d9ba 100644
--- a/js/ui/screenShield.js
+++ b/js/ui/screenShield.js
@@ -711,7 +711,7 @@ const ScreenShield = new Lang.Class({
         let unitaryDelay = ARROW_ANIMATION_TIME / (arrows.length + 1);
         let maxOpacity = 255 * ARROW_ANIMATION_PEAK_OPACITY;
         for (let i = 0; i < arrows.length; i++) {
-            arrows.opacity = 0;
+            arrows[i].opacity = 0;
             Tweener.addTween(arrows[i],
                              { opacity: 0,
                                delay: unitaryDelay * (N_ARROWS - (i + 1)),