Force an 'any' type for the hostInstance.parentNode in warning check

**what is the change?:**
This is awful. :(
I'm not sure how else to let Flow know that we expect that this might be
a sort of `DOMContainer` type and not just a normal `Node` type.

To at least make the type information clear we added a comment.

**why make this change?:**
To get `flow` passing. Looks like we have `any` types sprinkled
throughout this file. phooey. :(

**test plan:**
`yarn flow`

**issue:**
facebook#8854

src/renderers/dom/fiber/ReactDOMFiberEntry.js

@@ -75,11 +75,9 @@ findDOMNode._injectFiber(function(fiber: Fiber) {
 type DOMContainer =
   | (Element & {
     _reactRootContainer: ?Object,
-    __reactInternalIsPortalContainer: ?boolean,
   })
   | (Document & {
     _reactRootContainer: ?Object,
-    __reactInternalIsPortalContainer: ?boolean,
   });
 
 type Container = Element | Document;
@@ -541,10 +539,11 @@ function renderSubtreeIntoContainer(
       const hostInstance = DOMRenderer.findHostInstance(
         container._reactRootContainer.current,
       );
+      const hostInstanceParentNode: any =
+        hostInstance && hostInstance.parentNode;
       const hostInstanceParentIsPortal =
-        hostInstance &&
-        hostInstance.parentNode &&
-        hostInstance.parentNode.__reactInternalIsPortalContainer;
+        hostInstanceParentNode &&
+        hostInstanceParentNode.__reactInternalIsPortalContainer;
       if (hostInstance && !hostInstanceParentIsPortal) {
         warning(
           hostInstance.parentNode === container,