Fix: Initialize childIndex in Task constructor (facebook#27367)

This field was not being initialized. Although the property is part of
the Flow type, the type error wasn't caught because the constructor
itself is not covered by Flow, which is unfortunate. (I assume this is
related to the dev-only componentStack property.)

packages/react-server/src/ReactFizzServer.js

@@ -578,6 +578,7 @@ function createTask(
     context,
     treeContext,
     thenableState,
+    childIndex: -1,
   }: any);
   if (__DEV__) {
     task.componentStack = null;