Make assignedSlot return null for a slot in a closed shadow tree

third_party/WebKit/LayoutTests/fast/dom/shadow/resources/shadow-dom.js

 function createShadowRoot()
 {
     var children = Array.prototype.slice.call(arguments);
     if ((children[0] instanceof Object) && !(children[0] instanceof Node))
         return attachShadow.apply(null, children);
     return {'isShadowRoot': true,
             'children': children};
 }
 
 // TODO(kochi): This is not pure attachShadow wrapper, but also handles createShadowRoot()
@@ skipping 77 matching lines @@
         var template = nodes[i];
         var mode = template.getAttribute("data-mode");
         var parent = template.parentNode;
         parent.removeChild(template);
         var shadowRoot;
         if (!mode) {
             shadowRoot = parent.createShadowRoot();
         } else {
             shadowRoot = parent.attachShadow({'mode': mode});
         }
+        var expose = template.getAttribute("data-expose-as");
+        if (expose)
+            window[expose] = shadowRoot;
         if (template.id)
             shadowRoot.id = template.id;
         var fragments = document.importNode(template.content, true);
         shadowRoot.appendChild(fragments);
 
         convertTemplatesToShadowRootsWithin(shadowRoot);
     }
 }
 
 function isShadowHost(node)
@@ skipping 243 matching lines @@
                 return node;
         }
 
         return null;
     };
 
     if (!window.internals)
         return null;
     return iter(root, id);
 }