webdriver: remove ES6 constructs from pages (#9600)

Internet Explorer doesn't support ES6 constructs like fat arrow functions.
So that the WebDriver tests can be executed against IE, the pages loaded
in the browser by the tests shouldn't include such constructs.
diff --git a/webdriver/tests/element_click/bubbling.py b/webdriver/tests/element_click/bubbling.py
index 1cfb2ef..7bf405c 100644
--- a/webdriver/tests/element_click/bubbling.py
+++ b/webdriver/tests/element_click/bubbling.py
@@ -1,7 +1,6 @@
 from tests.support.asserts import assert_success
 from tests.support.inline import inline
 
-
 def click(session, element):
     return session.transport.send(
         "POST", "/session/{session_id}/element/{element_id}/click".format(
@@ -28,9 +27,10 @@
         <script>
         window.clicks = [];
 
-        for (let level of document.querySelectorAll("div")) {
-          level.addEventListener("click", ({currentTarget}) => {
-            window.clicks.push(currentTarget);
+        var elements = document.querySelectorAll("div");
+        for (var level = 0; level < elements.length; level++) {
+          elements[level].addEventListener("click", function(clickEvent) {
+            window.clicks.push(clickEvent.currentTarget);
           });
         }
         </script>
@@ -67,9 +67,11 @@
         <script>
         window.delayedClicks = [];
 
-        for (let level of document.querySelectorAll("div")) {
-          level.addEventListener("click", ({currentTarget}) => {
-            setTimeout(() => window.delayedClicks.push(currentTarget), 100);
+        var elements = document.querySelectorAll("div");
+        for (var level = 0; level < elements.length; level++) {
+          elements[level].addEventListener("click", function(clickEvent) {
+            var target = clickEvent.currentTarget;
+            setTimeout(function() { window.delayedClicks.push(target); }, 100);
           });
         }
         </script>