Fork findHostInstance to make findHostInstanceWithNoPortals

**what is the change?:**
We need to get host instances, but filter out portals. There is not
currently a method for that.

**why make this change?:**
Rather than change the existing `findHostInstance` method, which would
affect the behavior of the public `findDOMNode` method, we are forking.

**test plan:**
`yarn test`

**issue:**
facebook#8854

src/renderers/dom/fiber/ReactDOMFiberEntry.js

@@ -536,7 +536,7 @@ function renderSubtreeIntoContainer(
 
   if (__DEV__) {
     if (container._reactRootContainer && container.nodeType !== COMMENT_NODE) {
-      const hostInstance = DOMRenderer.findHostInstance(
+      const hostInstance = DOMRenderer.findHostInstanceWithNoPortals(
         container._reactRootContainer.current,
       );
       if (hostInstance) {

src/renderers/shared/fiber/ReactFiberReconciler.js

@@ -37,7 +37,10 @@ if (__DEV__) {
   var getComponentName = require('getComponentName');
 }
 
-var {findCurrentHostFiber} = require('ReactFiberTreeReflection');
+var {
+  findCurrentHostFiber,
+  findCurrentHostFiberWithNoPortals,
+} = require('ReactFiberTreeReflection');
 
 var getContextForSubtree = require('getContextForSubtree');
 
@@ -173,6 +176,9 @@ export type Reconciler<C, I, TI> = {
 
   // Use for findDOMNode/findHostNode. Legacy API.
   findHostInstance(component: Fiber): I | TI | null,
+
+  // Used internally for filtering out portals. Legacy API.
+  findHostInstanceWithNoPortals(component: Fiber): I | TI | null,
 };
 
 getContextForSubtree._injectFiber(function(fiber: Fiber) {
@@ -309,5 +315,13 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
       }
       return hostFiber.stateNode;
     },
+
+    findHostInstanceWithNoPortals(fiber: Fiber): PI | null {
+      const hostFiber = findCurrentHostFiberWithNoPortals(fiber);
+      if (hostFiber === null) {
+        return null;
+      }
+      return hostFiber.stateNode;
+    },
   };
 };

src/renderers/shared/fiber/ReactFiberTreeReflection.js

@@ -237,6 +237,41 @@ exports.findCurrentHostFiber = function(parent: Fiber): Fiber | null {
     return null;
   }
 
+  // Next we'll drill down this component to find the first HostComponent/Text.
+  let node: Fiber = currentParent;
+  while (true) {
+    if (node.tag === HostComponent || node.tag === HostText) {
+      return node;
+    } else if (node.child) {
+      node.child.return = node;
+      node = node.child;
+      continue;
+    }
+    if (node === currentParent) {
+      return null;
+    }
+    while (!node.sibling) {
+      if (!node.return || node.return === currentParent) {
+        return null;
+      }
+      node = node.return;
+    }
+    node.sibling.return = node.return;
+    node = node.sibling;
+  }
+  // Flow needs the return null here, but ESLint complains about it.
+  // eslint-disable-next-line no-unreachable
+  return null;
+};
+
+exports.findCurrentHostFiberWithNoPortals = function(
+  parent: Fiber,
+): Fiber | null {
+  const currentParent = findCurrentFiberUsingSlowPath(parent);
+  if (!currentParent) {
+    return null;
+  }
+
   // Next we'll drill down this component to find the first HostComponent/Text.
   let node: Fiber = currentParent;
   while (true) {