fix test, add comment for supporting ShadowDom v1

iron-focusables-helper.html

@@ -113,6 +113,19 @@
           result.push(element);
         }
 
+        // In ShadowDOM v1, tab order is affected by the order of distrubution.
+        // E.g. getTabbableNodes(#root) in ShadowDOM v1 should return [#A, #B];
+        // in ShadowDOM v0 tab order is not affected by the distrubution order,
+        // in fact getTabbableNodes(#root) returns [#B, #A].
+        //  <div id="root">
+        //   <!-- shadow -->
+        //     <slot name="a">
+        //     <slot name="b">
+        //   <!-- /shadow -->
+        //   <input id="A" slot="a">
+        //   <input id="B" slot="b" tabindex="1">
+        //  </div>
+        // TODO(valdrin) implement ShadowDOM v1 when upgrading to Polymer v2.0.
         var children;
         if (element.localName === 'content') {
           children = Polymer.dom(element).getDistributedNodes();

test/iron-overlay-behavior.html

@@ -677,8 +677,8 @@ <h2>Focusables (with tabindex)</h2>
 
         test('with-backdrop: after open, update last focusable node and then Shift+TAB', function(done) {
           overlay.withBackdrop = true;
-          var focusableNodes = overlay._focusableNodes;
           runAfterOpen(overlay, function() {
+            var focusableNodes = overlay._focusableNodes;
             // 1ms timeout needed by IE10 to have proper focus switching.
             Polymer.Base.async(function() {
               // Before tabbing, make lastFocusable non-tabbable. This will make