Index: frontend/node_modules/babel-preset-react-app/index.js
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--- frontend/node_modules/babel-preset-react-app/index.js	(revision 9af201e94b5a79beb92a30858fc54de4bc3b9449)
+++ frontend/node_modules/babel-preset-react-app/index.js	(revision 9af201e94b5a79beb92a30858fc54de4bc3b9449)
@@ -0,0 +1,20 @@
+/**
+ * Copyright (c) 2015-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';
+
+const create = require('./create');
+
+module.exports = function (api, opts) {
+  // This is similar to how `env` works in Babel:
+  // https://babeljs.io/docs/usage/babelrc/#env-option
+  // We are not using `env` because it’s ignored in versions > babel-core@6.10.4:
+  // https://github.com/babel/babel/issues/4539
+  // https://github.com/facebook/create-react-app/issues/720
+  // It’s also nice that we can enforce `NODE_ENV` being specified.
+  const env = process.env.BABEL_ENV || process.env.NODE_ENV;
+  return create(api, opts, env);
+};