new-webpack: functionize (#564)

Summary:
This will enable us to differentiate the production and development
behavior where necessary (primarily, only running minification in prod).

Best reviewed with `git show -w`.

Test Plan:
The diff with `git show -w` and the fact that `yarn flow` passes should
be sufficient. If you really want to be thorough, run Webpack with this
config file and `NODE_ENV` set to `production`.

wchargin-branch: webpack-functionize

config/makeWebpackConfig.js

@@ -30,7 +30,8 @@ if (env.stringified["process.env"].NODE_ENV !== '"production"') {
 // This is the production configuration.
 // It compiles slowly and is focused on producing a fast and minimal bundle.
 // The development configuration is different and lives in a separate file.
-module.exports = {
+function makeConfig(_unused_mode /*: "production" | "development" */) {
+  return {
     // Don't attempt to continue if there are any errors.
     bail: true,
     // We generate sourcemaps in production. This is slow but gives good results.
@@ -39,7 +40,10 @@ module.exports = {
     // In production, we only want to load the polyfills and the app code.
     entry: {
       main: [require.resolve("./polyfills"), paths.appIndexJs],
-    ssr: [require.resolve("./polyfills"), paths.appServerSideRenderingIndexJs],
+      ssr: [
+        require.resolve("./polyfills"),
+        paths.appServerSideRenderingIndexJs,
+      ],
     },
     output: {
       // The build folder.
@@ -261,3 +265,14 @@ module.exports = {
       child_process: "empty",
     },
   };
+}
+
+function getMode() {
+  const mode = process.env.NODE_ENV;
+  if (mode !== "production" && mode !== "development") {
+    throw new Error("unknown mode: " + String(mode));
+  }
+  return mode;
+}
+
+module.exports = makeConfig(getMode());