fix undefined dom.contains in IE11

Author: kmsheng

src/components/MdMenu/MdMenuContent.vue

@@ -201,7 +201,13 @@
         return this.$refs.menu ? this.$refs.menu.contains(target) : false
       },
       isBackdropExpectMenu ($event) {
-        return !this.$el.contains($event.target) && !this.isMenu($event)
+        const contains = (el, target) => {
+          if ('contains' in el) {
+            return el.contains(target)
+          }
+          return el.compareDocumentPosition(target) & 16
+        }
+        return !contains(this.$el, $event.target) && !this.isMenu($event)
       },
       createClickEventObserver () {
         if (document) {