Fork `babel-preset-react-app` into config/ (#212)

Summary:
We want to change this configuration so that our compilation of backend
applications can target latest Node. This commit forks the current
configuration so that we can modify it easily.

Test Plan:
Both `yarn start` and `yarn travis` work. The generated backend
applications work, too.

wchargin-branch: fork-babel-config

config/babel.js

@@ -0,0 +1,142 @@
+// @flow
+
+/**
+ * Copyright (c) 2018-present, SourceCred Authors.
+ * Copyright (c) 2015-2018, Facebook, Inc.
+ *
+ * Forked from babel-preset-react-app/index.js, which is released under
+ * the MIT license.
+ */
+
+const plugins = [
+  // Necessary to include regardless of the environment because
+  // in practice some other transforms (such as object-rest-spread)
+  // don't work without it: https://github.com/babel/babel/issues/7215
+  require.resolve("babel-plugin-transform-es2015-destructuring"),
+  // class { handleClick = () => { } }
+  require.resolve("babel-plugin-transform-class-properties"),
+  // The following two plugins use Object.assign directly, instead of Babel's
+  // extends helper. Note that this assumes `Object.assign` is available.
+  // { ...todo, completed: true }
+  [
+    require.resolve("babel-plugin-transform-object-rest-spread"),
+    {
+      useBuiltIns: true,
+    },
+  ],
+  // Transforms JSX
+  [
+    require.resolve("babel-plugin-transform-react-jsx"),
+    {
+      useBuiltIns: true,
+    },
+  ],
+  // Polyfills the runtime needed for async/await and generators
+  [
+    require.resolve("babel-plugin-transform-runtime"),
+    {
+      helpers: false,
+      polyfill: false,
+      regenerator: true,
+    },
+  ],
+];
+
+// 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/facebookincubator/create-react-app/issues/720
+// It’s also nice that we can enforce `NODE_ENV` being specified.
+var env = process.env.BABEL_ENV || process.env.NODE_ENV;
+if (env !== "development" && env !== "test" && env !== "production") {
+  throw new Error(
+    "Using `babel-preset-react-app` requires that you specify `NODE_ENV` or " +
+      '`BABEL_ENV` environment variables. Valid values are "development", ' +
+      '"test", and "production". Instead, received: ' +
+      JSON.stringify(env) +
+      "."
+  );
+}
+
+if (env === "development" || env === "test") {
+  // The following two plugins are currently necessary to make React warnings
+  // include more valuable information. They are included here because they are
+  // currently not enabled in babel-preset-react. See the below threads for more info:
+  // https://github.com/babel/babel/issues/4702
+  // https://github.com/babel/babel/pull/3540#issuecomment-228673661
+  // https://github.com/facebookincubator/create-react-app/issues/989
+  plugins.push.apply(plugins, [
+    // Adds component stack to warning messages
+    require.resolve("babel-plugin-transform-react-jsx-source"),
+    // Adds __self attribute to JSX which React will use for some warnings
+    require.resolve("babel-plugin-transform-react-jsx-self"),
+  ]);
+}
+
+if (env === "test") {
+  module.exports = {
+    presets: [
+      // ES features necessary for user's Node version
+      [
+        require("babel-preset-env").default,
+        {
+          targets: {
+            node: "current",
+          },
+        },
+      ],
+      // JSX, Flow
+      require.resolve("babel-preset-react"),
+    ],
+    plugins: plugins.concat([
+      // Compiles import() to a deferred require()
+      require.resolve("babel-plugin-dynamic-import-node"),
+    ]),
+  };
+} else {
+  module.exports = {
+    presets: [
+      // Latest stable ECMAScript features
+      [
+        require.resolve("babel-preset-env"),
+        {
+          targets: {
+            // React parses on ie 9, so we should too
+            ie: 9,
+            // We currently minify with uglify
+            // Remove after https://github.com/mishoo/UglifyJS2/issues/448
+            uglify: true,
+          },
+          // Disable polyfill transforms
+          useBuiltIns: false,
+          // Do not transform modules to CJS
+          modules: false,
+        },
+      ],
+      // JSX, Flow
+      require.resolve("babel-preset-react"),
+    ],
+    plugins: plugins.concat([
+      // function* () { yield 42; yield 43; }
+      [
+        require.resolve("babel-plugin-transform-regenerator"),
+        {
+          // Async functions are converted to generators by babel-preset-env
+          async: false,
+        },
+      ],
+      // Adds syntax support for import()
+      require.resolve("babel-plugin-syntax-dynamic-import"),
+    ]),
+  };
+
+  if (env === "production") {
+    // Optimization: hoist JSX that never changes out of render()
+    // Disabled because of issues: https://github.com/facebookincubator/create-react-app/issues/553
+    // TODO: Enable again when these issues are resolved.
+    // plugins.push.apply(plugins, [
+    //   require.resolve('babel-plugin-transform-react-constant-elements')
+    // ]);
+  }
+}

package.json

@@ -110,7 +110,7 @@
       "flow-react-proptypes"
     ],
     "presets": [
-      "react-app"
+      "./config/babel"
     ]
   },
   "devDependencies": {