utils.js: fix mozjs52 warning

This commit prevent the warning message (#586):

"reference to undefined property Symbol.toPrimitive".

It seems that gnome-shell 3.26 (or mozjs52) doesn't like the use of
direct method calls on map elements, although we don't know why.

utils.js

@@ -31,7 +31,8 @@ const BasicHandler = new Lang.Class({
 
         // Skip first element of the arguments
         for (let i = 1; i < arguments.length; i++) {
-            this._storage[label].push( this._create(arguments[i]));
+            let item = this._storage[label];
+            item.push(this._create(arguments[i]));
         }
     },
 
@@ -251,4 +252,4 @@ function drawRoundedLine(cr, x, y, width, height, isRoundLeft, isRoundRight, str
     if (stroke != null)
         cr.setSource(stroke);
     cr.stroke();
-}
+}