Should be able to unmount an error boundary before it is handled

Fixes the case where an error boundary captures an error, but its
parent is unmounted before we can re-render it. componentDidCatch is
never called, and we don't remove the boundary from our set of
unhandled error boundaries.

We should not assume that if capturedErrors is non-null that we still
have unhandled errors.

src/renderers/shared/fiber/ReactFiberScheduler.js

@@ -761,7 +761,11 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
     // The loop stops once the children have unmounted and error lifecycles are
     // called. Then we return to the regular flow.
 
-    if (capturedErrors !== null && capturedErrors.size > 0) {
+    if (
+      capturedErrors !== null &&
+      capturedErrors.size > 0 &&
+      nextPriorityLevel === TaskPriority
+    ) {
       while (nextUnitOfWork !== null) {
         if (hasCapturedError(nextUnitOfWork)) {
           // Use a forked version of performUnitOfWork
@@ -780,19 +784,17 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
           commitAllWork(pendingCommit);
           priorityContext = nextPriorityLevel;
 
-          if (capturedErrors === null || capturedErrors.size === 0) {
+          if (
+            capturedErrors === null ||
+            capturedErrors.size === 0 ||
+            nextPriorityLevel !== TaskPriority
+          ) {
             // There are no more unhandled errors. We can exit this special
             // work loop. If there's still additional work, we'll perform it
             // using one of the normal work loops.
             break;
           }
           // The commit phase produced additional errors. Continue working.
-          invariant(
-            nextPriorityLevel === TaskPriority,
-            'Commit phase errors should be scheduled to recover with task ' +
-              'priority. This error is likely caused by a bug in React. ' +
-              'Please file an issue.',
-          );
         }
       }
     }

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

@@ -512,6 +512,42 @@ describe('ReactIncrementalErrorHandling', () => {
     ReactNoop.render(<Parent />);
   });
 
+  it('can unmount an error boundary before it is handled', () => {
+    let parent;
+
+    class Parent extends React.Component {
+      state = {step: 0};
+      render() {
+        parent = this;
+        return this.state.step === 0 ? <Boundary /> : null;
+      }
+    }
+
+    class Boundary extends React.Component {
+      componentDidCatch() {}
+      render() {
+        return <Child />;
+      }
+    }
+
+    class Child extends React.Component {
+      componentDidUpdate() {
+        parent.setState({step: 1});
+        throw new Error('update error');
+      }
+      render() {
+        return null;
+      }
+    }
+
+    ReactNoop.render(<Parent />);
+    ReactNoop.flush();
+
+    ReactNoop.flushSync(() => {
+      ReactNoop.render(<Parent />);
+    });
+  });
+
   it('continues work on other roots despite caught errors', () => {
     class ErrorBoundary extends React.Component {
       state = {error: null};