Tighten up the types of getFirstHydratableChild

We currently call getFirstHydratableChild to step into the children of
a suspense boundary. This can be a text node or a suspense boundary
which isn't compatible with getFirstHydratableChild, and we cheat the type.

This accidentally works because .firstChild always returns null on those
nodes in the DOM.

This just makes that explicit.

packages/react-reconciler/src/ReactFiberHydrationContext.new.js

@@ -261,6 +261,8 @@ function tryHydrate(fiber, nextInstance) {
       const instance = canHydrateInstance(nextInstance, type, props);
       if (instance !== null) {
         fiber.stateNode = (instance: Instance);
+        hydrationParentFiber = fiber;
+        nextHydratableInstance = getFirstHydratableChild(instance);
         return true;
       }
       return false;
@@ -270,6 +272,9 @@ function tryHydrate(fiber, nextInstance) {
       const textInstance = canHydrateTextInstance(nextInstance, text);
       if (textInstance !== null) {
         fiber.stateNode = (textInstance: TextInstance);
+        hydrationParentFiber = fiber;
+        // Text Instances don't have children so there's nothing to hydrate.
+        nextHydratableInstance = null;
         return true;
       }
       return false;
@@ -294,6 +299,10 @@ function tryHydrate(fiber, nextInstance) {
           );
           dehydratedFragment.return = fiber;
           fiber.child = dehydratedFragment;
+          hydrationParentFiber = fiber;
+          // While a Suspense Instance does have children, we won't step into
+          // it during the first pass. Instead, we'll reenter it later.
+          nextHydratableInstance = null;
           return true;
         }
       }
@@ -338,8 +347,6 @@ function tryToClaimNextHydratableInstance(fiber: Fiber): void {
       firstAttemptedInstance,
     );
   }
-  hydrationParentFiber = fiber;
-  nextHydratableInstance = getFirstHydratableChild((nextInstance: any));
 }
 
 function prepareToHydrateHostInstance(

packages/react-reconciler/src/ReactFiberHydrationContext.old.js

@@ -261,6 +261,8 @@ function tryHydrate(fiber, nextInstance) {
       const instance = canHydrateInstance(nextInstance, type, props);
       if (instance !== null) {
         fiber.stateNode = (instance: Instance);
+        hydrationParentFiber = fiber;
+        nextHydratableInstance = getFirstHydratableChild(instance);
         return true;
       }
       return false;
@@ -270,6 +272,9 @@ function tryHydrate(fiber, nextInstance) {
       const textInstance = canHydrateTextInstance(nextInstance, text);
       if (textInstance !== null) {
         fiber.stateNode = (textInstance: TextInstance);
+        hydrationParentFiber = fiber;
+        // Text Instances don't have children so there's nothing to hydrate.
+        nextHydratableInstance = null;
         return true;
       }
       return false;
@@ -294,6 +299,10 @@ function tryHydrate(fiber, nextInstance) {
           );
           dehydratedFragment.return = fiber;
           fiber.child = dehydratedFragment;
+          hydrationParentFiber = fiber;
+          // While a Suspense Instance does have children, we won't step into
+          // it during the first pass. Instead, we'll reenter it later.
+          nextHydratableInstance = null;
           return true;
         }
       }
@@ -338,8 +347,6 @@ function tryToClaimNextHydratableInstance(fiber: Fiber): void {
       firstAttemptedInstance,
     );
   }
-  hydrationParentFiber = fiber;
-  nextHydratableInstance = getFirstHydratableChild((nextInstance: any));
 }
 
 function prepareToHydrateHostInstance(