Add test for errors in host config while working on failed root

Test for a bug fixed in the previous commit that was causing an
infinite loop.

scripts/fiber/tests-passing.txt

@@ -1183,6 +1183,7 @@ src/renderers/shared/fiber/__tests__/ReactIncrementalErrorHandling-test.js
 * recovers from uncaught reconciler errors
 * unmounts components with uncaught errors
 * does not interrupt unmounting if detaching a ref throws
+* handles error thrown by host config while working on failed root
 
 src/renderers/shared/fiber/__tests__/ReactIncrementalReflection-test.js
 * handles isMounted even when the initial render is deferred

src/renderers/noop/ReactNoop.js

@@ -39,9 +39,14 @@ type TextInstance = {| text: string, id: number |};
 
 var instanceCounter = 0;
 
+var failInBeginPhase = false;
+
 var NoopRenderer = ReactFiberReconciler({
 
   getRootHostContext() {
+    if (failInBeginPhase) {
+      throw new Error('Error in host config.');
+    }
     return emptyObject;
   },
 
@@ -353,6 +358,15 @@ var ReactNoop = {
     console.log(...bufferedLog);
   },
 
+  simulateErrorInHostConfig(fn : () => void) {
+    failInBeginPhase = true;
+    try {
+      fn();
+    } finally {
+      failInBeginPhase = false;
+    }
+  },
+
 };
 
 module.exports = ReactNoop;

src/renderers/shared/fiber/__tests__/ReactIncrementalErrorHandling-test.js

@@ -917,4 +917,11 @@ describe('ReactIncrementalErrorHandling', () => {
     // Because there was an error, entire tree should unmount
     expect(ReactNoop.getChildren()).toEqual([]);
   });
+
+  it('handles error thrown by host config while working on failed root', () => {
+    ReactNoop.simulateErrorInHostConfig(() => {
+      ReactNoop.render(<span />);
+      expect(() => ReactNoop.flush()).toThrow('Error in host config.');
+    });
+  });
 });