Add Flowtests for HostComponent

Summary:
`HostComponent` is built specifically to differentiate from `ReactNative.NativeComponent`. These tests should ensure that is the case, and help it stay that way.

I also expect these tests to be duplicated to DefinitelyTyped to help the team working on the TypeScript types ensure they have things modeled correctly.

Reviewed By: cpojer

Differential Revision: D17580120

fbshipit-source-id: c14eb18507cbee1b308beeff0092607e18706171

Libraries/Renderer/__flowtests__/ReactNativeTypes-flowtest.js

@@ -0,0 +1,52 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
+ * @format
+ */
+
+'use strict';
+
+import * as React from 'react';
+import type {
+  HostComponent,
+  ReactNativeComponentClass_ForTestsOnly,
+} from '../shims/ReactNativeTypes';
+
+function takesHostComponentInstance(
+  instance: React$ElementRef<HostComponent<mixed>> | null,
+): void {}
+
+const MyHostComponent = (('Host': any): HostComponent<mixed>);
+
+<MyHostComponent
+  ref={hostComponentRef => {
+    takesHostComponentInstance(hostComponentRef);
+
+    if (hostComponentRef == null) {
+      return;
+    }
+
+    hostComponentRef.measureLayout(hostComponentRef, () => {});
+  }}
+/>;
+
+declare var NativeComponent: ReactNativeComponentClass_ForTestsOnly<{}>;
+class MyNativeComponent extends NativeComponent {}
+
+<MyNativeComponent
+  ref={nativeComponentRef => {
+    // $FlowExpectedError - NativeComponent cannot be passed as HostComponent.
+    takesHostComponentInstance(nativeComponentRef);
+
+    if (nativeComponentRef == null) {
+      return;
+    }
+
+    // $FlowExpectedError - NativeComponent cannot be passed as HostComponent.
+    nativeComponentRef.measureLayout(nativeComponentRef, () => {});
+  }}
+/>;

Libraries/Renderer/shims/ReactNativeTypes.js

@@ -97,6 +97,11 @@ class ReactNativeComponent<Props> extends React.Component<Props> {
   setNativeProps(nativeProps: Object): void {}
 }
 
+// This type is only used for FlowTests. It shouldn't be imported directly
+export type ReactNativeComponentClass_ForTestsOnly<Props> = Class<
+  ReactNativeComponent<Props>,
+>;
+
 /**
  * This type keeps HostComponent and NativeMethodsMixin in sync.
  * It can also provide types for ReactNative applications that use NMM or refs.