[RFC] Add react-is package

packages/react-is/README.md

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+# `react-is`
+
+This package allows you to test arbitrary values and see if they're a particular React type, e.g. React Elements.
+
+## Usage
+
+```js
+import React from 'react';
+import {isElement} from 'react-is';
+isElement(<div />); // true
+```
+
+```js
+import React from 'react';
+import {isFragment} from 'react-is';
+isFragment(<></>); // true
+```
+
+```js
+import React from 'react';
+import {createPortal} from 'react-dom';
+import {isPortal} from 'react-is';
+isPortal(createPortal(<div />, document.body)); // true
+```
+
+```js
+import React from 'react';
+import {createPortal} from 'react-dom';
+import {typeOf} from 'react-is';
+typeOf(<div />); // "ReactElement"
+typeOf(<></>); // "ReactFragment"
+typeOf(createPortal(<div />, document.body)); // "ReactPortal"
+```

packages/react-is/index.js

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+/**
+ * Copyright (c) 2014-present, Facebook, Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+'use strict';
+
+var REACT_ELEMENT_TYPE;
+var REACT_COROUTINE_TYPE;
+var REACT_YIELD_TYPE;
+var REACT_PORTAL_TYPE;
+var REACT_FRAGMENT_TYPE;
+if (typeof Symbol === 'function' && Symbol.for) {
+  REACT_ELEMENT_TYPE = Symbol.for('react.element');
+  REACT_COROUTINE_TYPE = Symbol.for('react.coroutine');
+  REACT_YIELD_TYPE = Symbol.for('react.yield');
+  REACT_PORTAL_TYPE = Symbol.for('react.portal');
+  REACT_FRAGMENT_TYPE = Symbol.for('react.fragment');
+} else {
+  REACT_ELEMENT_TYPE = 0xeac7;
+  REACT_COROUTINE_TYPE = 0xeac8;
+  REACT_YIELD_TYPE = 0xeac9;
+  REACT_PORTAL_TYPE = 0xeaca;
+  REACT_FRAGMENT_TYPE = 0xeacb;
+}
+
+function is(object, type) {
+  return (
+    typeof object === 'object' && object !== null && object.$$typeof === type
+  );
+}
+
+module.exports = {
+  typeOf(object) {
+    switch (typeof object === 'object' && object !== null && object.$$typeof) {
+      case REACT_ELEMENT_TYPE:
+        return 'ReactElement';
+      case REACT_COROUTINE_TYPE:
+        return 'ReactCoroutine';
+      case REACT_YIELD_TYPE:
+        return 'ReactYield';
+      case REACT_PORTAL_TYPE:
+        return 'ReactPortal';
+      case REACT_FRAGMENT_TYPE:
+        return 'ReactFragment';
+      default:
+        return undefined;
+    }
+  },
+  isElement(object) {
+    return is(object, REACT_ELEMENT_TYPE);
+  },
+  isCoroutine(object) {
+    return is(object, REACT_COROUTINE_TYPE);
+  },
+  isYield(object) {
+    return is(object, REACT_YIELD_TYPE);
+  },
+  isPortal(object) {
+    return is(object, REACT_PORTAL_TYPE);
+  },
+  isFragment(object) {
+    return is(object, REACT_FRAGMENT_TYPE);
+  },
+};

packages/react-is/package.json

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+{
+  "name": "react-is",
+  "description": "Brand checking of React Elements.",
+  "keywords": ["react"],
+  "version": "1.0.0",
+  "homepage": "https://reactjs.org/",
+  "bugs": "https://github.com/facebook/react/issues",
+  "license": "MIT",
+  "files": ["index.js"],
+  "main": "index.js",
+  "repository": "facebook/react"
+}