Add two polyfills for nextElementSibling and previousElementSibling

I've added two new files polyfill files although they didn't come directly
from polyfill.io they were based off a PR that was merged in the library
but not included in the new polyfill-library repo.
I've added comments pointing to the original pull request for the
detection and for the polyfill.

src/components/tabs/tabs.js

@@ -1,5 +1,7 @@
 import '../../vendor/polyfills/Function/prototype/bind'
 import '../../vendor/polyfills/Element/prototype/classList'
+import '../../vendor/polyfills/Element/prototype/nextElementSibling'
+import '../../vendor/polyfills/Element/prototype/previousElementSibling'
 import '../../vendor/polyfills/Event' // addEventListener and event.target normaliziation
 import { nodeListForEach } from '../../common'
 
@@ -205,7 +207,7 @@ Tabs.prototype.onTabKeydown = function (e) {
 
 Tabs.prototype.activateNextTab = function () {
   var currentTab = this.getCurrentTab()
-  var nextTabListItem = currentTab.parentNode.nextSibling
+  var nextTabListItem = currentTab.parentNode.nextElementSibling
   if (nextTabListItem) {
     var nextTab = nextTabListItem.querySelector('.govuk-tabs__tab')
   }
@@ -219,7 +221,7 @@ Tabs.prototype.activateNextTab = function () {
 
 Tabs.prototype.activatePreviousTab = function () {
   var currentTab = this.getCurrentTab()
-  var previousTabListItem = currentTab.parentNode.previousSibling
+  var previousTabListItem = currentTab.parentNode.previousElementSibling
   if (previousTabListItem) {
     var previousTab = previousTabListItem.querySelector('.govuk-tabs__tab')
   }

src/vendor/polyfills/Element/prototype/nextElementSibling.js

@@ -0,0 +1,44 @@
+import '../../Object/defineProperty'
+import '../../Element'
+
+(function(undefined) {
+
+    // Detection from https://github.com/Financial-Times/polyfill-service/pull/1062/files#diff-b09a5d2acf3314b46a6c8f8d0c31b85c
+    var detect = (
+      'Element' in this && "nextElementSibling" in document.documentElement
+    )
+
+    if (detect) return
+
+
+    (function (global) {
+      var dpSupport = true;
+      var defineGetter = function (object, name, fn, configurable) {
+        if (Object.defineProperty)
+          Object.defineProperty(object, name, {
+            configurable: false === dpSupport ? true : !!configurable,
+            get: fn
+          });
+
+        else object.__defineGetter__(name, fn);
+      };
+      /** Ensure the browser allows Object.defineProperty to be used on native JavaScript objects. */
+      try {
+        defineGetter({}, "support");
+      }
+      catch (e) {
+        dpSupport = false;
+      }
+
+      // Polyfill from https://github.com/Financial-Times/polyfill-service/pull/1062/files#diff-404b69b4750d18dea4174930a49170fd
+      Object.defineProperty(Element.prototype, "nextElementSibling", {
+        get: function(){
+          var el = this.nextSibling;
+          while (el && el.nodeType !== 1) { el = el.nextSibling; }
+          return (el.nodeType === 1) ? el : null;
+        }
+      });
+
+    }(this));
+
+}).call('object' === typeof window && window || 'object' === typeof self && self || 'object' === typeof global && global || {});

src/vendor/polyfills/Element/prototype/previousElementSibling.js

@@ -0,0 +1,43 @@
+import '../../Object/defineProperty'
+import '../../Element'
+
+(function(undefined) {
+
+    // Detection from https://github.com/Financial-Times/polyfill-service/pull/1062/files#diff-a162235fbc9c0dd40d4032265f44942e
+    var detect = (
+      'Element' in this && 'previousElementSibling' in document.documentElement
+    )
+
+    if (detect) return
+
+    (function (global) {
+      var dpSupport = true;
+      var defineGetter = function (object, name, fn, configurable) {
+        if (Object.defineProperty)
+          Object.defineProperty(object, name, {
+            configurable: false === dpSupport ? true : !!configurable,
+            get: fn
+          });
+
+        else object.__defineGetter__(name, fn);
+      };
+      /** Ensure the browser allows Object.defineProperty to be used on native JavaScript objects. */
+      try {
+        defineGetter({}, "support");
+      }
+      catch (e) {
+        dpSupport = false;
+      }
+
+      // Polyfill from https://github.com/Financial-Times/polyfill-service/pull/1062/files#diff-b45a1197b842728cb76b624b6ba7d739
+      Object.defineProperty(Element.prototype, 'previousElementSibling', {
+        get: function(){
+          var el = this.previousSibling;
+          while (el && el.nodeType !== 1) { el = el.previousSibling; }
+          return (el.nodeType === 1) ? el : null;
+        }
+      });
+
+    }(this));
+
+}).call('object' === typeof window && window || 'object' === typeof self && self || 'object' === typeof global && global || {});