Fix type error and improve name of portal container flag

**NOTE:** I am still looking for a good place to move this flag
assignment to, or a better approach. This does some intermediate fixes.

**what is the change?:**
- fixed flow error by allowing optional flag on a DOMContainer that
  indicates it was used as a portal container.
- renamed the flag to something which makes more sense

**why make this change?:**
- get Flow passing
- make this change make more sense

We are still not sure about adding this flag; a follow-up diff may move
it or take a different approach.

**test plan:**
`yarn test`

**issue:**
#8854

Author: flarnie

src/renderers/dom/fiber/ReactDOMFiberEntry.js

@@ -73,8 +73,14 @@ findDOMNode._injectFiber(function(fiber: Fiber) {
 });
 
 type DOMContainer =
-  | (Element & {_reactRootContainer: ?Object})
-  | (Document & {_reactRootContainer: ?Object});
+  | (Element & {
+    _reactRootContainer: ?Object,
+    __reactInternalIsPortalContainer: ?boolean,
+  })
+  | (Document & {
+    _reactRootContainer: ?Object,
+    __reactInternalIsPortalContainer: ?boolean,
+  });
 
 type Container = Element | Document;
 type Props = {
@@ -538,7 +544,7 @@ function renderSubtreeIntoContainer(
       const hostInstanceParentIsPortal =
         hostInstance &&
         hostInstance.parentNode &&
-        hostInstance.parentNode._reactPortalContainer;
+        hostInstance.parentNode.__reactInternalIsPortalContainer;
       if (hostInstance && !hostInstanceParentIsPortal) {
         warning(
           hostInstance.parentNode === container,

src/renderers/shared/fiber/isomorphic/ReactPortal.js

@@ -27,7 +27,8 @@ exports.createPortal = function(
   implementation: any,
   key: ?string = null,
 ): ReactPortal {
-  containerInfo._reactPortalContainer = {};
+  // This flag allows us to check if a node was used with a portal
+  containerInfo.__reactInternalIsPortalContainer = true;
   return {
     // This tag allow us to uniquely identify this as a React Portal
     $$typeof: REACT_PORTAL_TYPE,