Add flag to portalContainer on mount instead of in createPortal

**what is the change?:**
We add a flag to the container of a 'portal' in the 'commit work' phase
in Fiber. This is right before we call `appendChildToContainer`.

**why make this change?:**
- Sometimes people call `ReactDOM.render(... container)`, then manually
clear the content of the `container`, and then try to call another
`ReactDOM.render(... container)`.
- This leads to cryptic errors or silent failure because we hold a
  reference to the node that was rendered the first time, and expect it
  to still be inside the container.
- We added a warning for this issue in `renderSubtreeIntoContainer`, but
  when a component renders something returned by
  `ReactDOM.unstable_createPortal(<Component />, portalContainer);`,
  then the child is inside the `portalContainer` and not the `container,
  but that is valid and we want to skip warning in that case.

Inside `renderSubtreeIntoContainer` we don't have the info to determine
if a child was rendered into a `portalContainer` or a `container`, and
adding this flag lets us figure that out and skip the warning.

We originally added the flag in the call to
`ReactDOM.unstable_createPortal` but that seemed like a method that
should be "pure" and free of side-effects. This commit moves the
flag-adding to happen when we mount the portal component.

**test plan:**
`yarn test`

**issue:**
facebook#8854

Author: flarnie

src/renderers/shared/fiber/ReactFiberCommitWork.js

@@ -218,6 +218,10 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
           }
         } else {
           if (isContainer) {
+            if (parentFiber.tag === HostPortal) {
+              // allows us to identify the portal container in other places
+              parent.__reactInternalIsPortalContainer = true;
+            }
             appendChildToContainer(parent, node.stateNode);
           } else {
             appendChild(parent, node.stateNode);

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

@@ -27,8 +27,6 @@ exports.createPortal = function(
   implementation: any,
   key: ?string = null,
 ): ReactPortal {
-  // 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,