Externalize React helpers.

React’s official policy on these helpers is “use at your own risk”:
- facebook/react#1906 (comment)
- facebook/react#2251 (comment)
- facebook/react#2317

package.json

@@ -24,7 +24,10 @@
   ],
   "dependencies": {
     "core-js": "^0.9.18",
-    "he": "^0.5.0"
+    "exenv": "^1.2.0",
+    "he": "^0.5.0",
+    "invariant": "^2.1.0",
+    "shallowequal": "^0.2.1"
   },
   "peerDependencies": {
     "react": "^0.13.x"

src/CreateSideEffect.jsx

@@ -1,6 +1,6 @@
 import React from "react";
-import invariant from "react/lib/invariant";
-import shallowEqual from "react/lib/shallowEqual";
+import invariant from "invariant";
+import shallowequal from "shallowequal";
 
 const RESERVED_PROPS = {
     arguments: true,
@@ -33,7 +33,7 @@ export default (Component) => {
         }
 
         shouldComponentUpdate(nextProps) {
-            return !shallowEqual(nextProps, this.props);
+            return !shallowequal(nextProps, this.props);
         }
 
         componentDidUpdate() {

src/Helmet.jsx

@@ -1,5 +1,5 @@
 import React from "react";
-import ExecutionEnvironment from "react/lib/ExecutionEnvironment";
+import ExecutionEnvironment from "exenv";
 import CreateSideEffect from "./CreateSideEffect";
 import {TAG_NAMES, TAG_PROPERTIES} from "./HelmetConstants.js";
 import HTMLEntities from "he";