Make contains() actually work in IE8

Summary: IE8 doesn't have indexOf on arrays!

Reviewers: emily

Reviewed By: emily

Differential Revision: http://phabricator.khanacademy.org/D3052

utils.js

@@ -1,7 +1,18 @@
-function contains(list, elem) {
+function fastContains(list, elem) {
     return list.indexOf(elem) !== -1;
 }
 
+function slowContains(list, elem) {
+    for (var i = 0; i < list.length; i++) {
+        if (list[i] === elem) {
+            return true;
+        }
+    }
+    return false;
+}
+
+var contains = Array.prototype.indexOf ? fastContains : slowContains;
+
 module.exports = {
     contains: contains
 }