Skip portals when looking for host siblings

A portal is not part of that host tree despite being a child.

scripts/fiber/tests-failing.txt

@@ -18,9 +18,6 @@ src/isomorphic/modern/element/__tests__/ReactJSXElementValidator-test.js
 src/renderers/dom/__tests__/ReactDOMProduction-test.js
 * should throw with an error code in production
 
-src/renderers/dom/fiber/__tests__/ReactDOMFiber-test.js
-* should unwind namespaces on caught errors in a portal
-
 src/renderers/dom/shared/__tests__/ReactDOM-test.js
 * throws in render() if the mount callback is not a function
 * throws in render() if the update callback is not a function

scripts/fiber/tests-passing.txt

@@ -519,6 +519,7 @@ src/renderers/dom/fiber/__tests__/ReactDOMFiber-test.js
 * should keep track of namespace across portals (complex)
 * should unwind namespaces on uncaught errors
 * should unwind namespaces on caught errors
+* should unwind namespaces on caught errors in a portal
 * should pass portal context when rendering subtree elsewhere
 * should update portal context if it changes due to setState
 * should update portal context if it changes due to re-render

src/renderers/shared/fiber/ReactFiberCommitWork.js

@@ -152,7 +152,7 @@ module.exports = function<T, P, I, TI, C, CX>(
           // If we don't have a child, try the siblings instead.
           continue siblings;
         }
-        if (!node.child) {
+        if (!node.child || node.tag === HostPortal) {
           continue siblings;
         } else {
           node.child.return = node;