Move isWorking assignment out of branches to top

packages/react-reconciler/src/ReactFiberScheduler.js

@@ -944,13 +944,13 @@ export default function<T, P, I, TI, HI, PI, C, CC, CX, PL>(
     } while (true);
 
     // We're done performing work. Time to clean up.
-    let didCompleteRoot = null;
+    let didCompleteRoot = false;
+    isWorking = false;
 
     // Yield back to main thread.
     if (didFatal) {
       stopWorkLoopTimer(interruptedBy, didCompleteRoot);
       interruptedBy = null;
-      isWorking = false;
       // There was a fatal error.
       if (__DEV__) {
         stack.resetStackAfterFatalErrorInDev();
@@ -962,7 +962,6 @@ export default function<T, P, I, TI, HI, PI, C, CC, CX, PL>(
         didCompleteRoot = true;
         stopWorkLoopTimer(interruptedBy, didCompleteRoot);
         interruptedBy = null;
-        isWorking = false;
         // The root successfully completed. It's ready for commit.
         root.pendingCommitExpirationTime = expirationTime;
         const finishedWork = root.current.alternate;
@@ -971,7 +970,6 @@ export default function<T, P, I, TI, HI, PI, C, CC, CX, PL>(
         // The root did not complete.
         stopWorkLoopTimer(interruptedBy, didCompleteRoot);
         interruptedBy = null;
-        isWorking = false;
         invariant(
           false,
           'Expired work should have completed. This error is likely caused ' +
@@ -981,7 +979,6 @@ export default function<T, P, I, TI, HI, PI, C, CC, CX, PL>(
     } else {
       stopWorkLoopTimer(interruptedBy, didCompleteRoot);
       interruptedBy = null;
-      isWorking = false;
       // There's more work to do, but we ran out of time. Yield back to
       // the renderer.
       return null;