[Fiber] Throw when attempting to unmount at an invalid node (#8688)

* Throw when attempting to unmount at an invalid node

* Remove extra character after invariant

scripts/fiber/tests-failing.txt

@@ -26,7 +26,6 @@ src/renderers/dom/shared/__tests__/ReactDOMTextComponent-test.js
 * can reconcile text arbitrarily split into multiple nodes on some substitutions only
 
 src/renderers/dom/shared/__tests__/ReactMount-test.js
-* throws when given a non-node
 * throws when given a string
 * throws when given a factory
 * tracks root instances

scripts/fiber/tests-passing.txt

@@ -752,6 +752,7 @@ src/renderers/dom/shared/__tests__/ReactEventListener-test.js
 * should not fire duplicate events for a React DOM tree
 
 src/renderers/dom/shared/__tests__/ReactMount-test.js
+* throws when given a non-node
 * should render different components in same root
 * should unmount and remount if the key changes
 * should reuse markup if rendering to the same target twice

src/renderers/dom/fiber/ReactDOMFiber.js

@@ -352,6 +352,10 @@ var ReactDOM = {
   },
 
   unmountComponentAtNode(container : DOMContainerElement) {
+    invariant(
+      isValidContainer(container),
+      'unmountComponentAtNode(...): Target container is not a DOM element.'
+    );
     warnAboutUnstableUse();
     if (container._reactRootContainer) {
       // Unmount should not be batched.