Extract some utilities into a separate package (#723)

* Extract some utilities into a separate package

* Add utils dir to `files` in package.json

* Do not create an empty `utils` dir on eject

packages/react-scripts/config/env.js

@@ -0,0 +1,38 @@
+// @remove-on-eject-begin
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+// @remove-on-eject-end
+
+// Grab NODE_ENV and REACT_APP_* environment variables and prepare them to be
+// injected into the application via DefinePlugin in Webpack configuration.
+
+var REACT_APP = /^REACT_APP_/i;
+
+function getClientEnvironment(publicUrl) {
+  return Object
+    .keys(process.env)
+    .filter(key => REACT_APP.test(key))
+    .reduce((env, key) => {
+      env['process.env.' + key] = JSON.stringify(process.env[key]);
+      return env;
+    }, {
+      // Useful for determining whether we’re running in production mode.
+      // Most importantly, it switches React into the correct mode.
+      'process.env.NODE_ENV': JSON.stringify(
+        process.env.NODE_ENV || 'development'
+      ),
+      // Useful for resolving the correct path to static assets in `public`.
+      // For example, <img src={process.env.PUBLIC_URL + '/img/logo.png'} />.
+      // This should only be used as an escape hatch. Normally you would put
+      // images into the `src` and `import` them in code to get their paths.
+      'process.env.PUBLIC_URL': JSON.stringify(publicUrl)
+    });
+}
+
+module.exports = getClientEnvironment;