new-webpack: only minify in prod (#566)

Summary:
Extraction of the plugin list to a function is mostly trivial, but
requires a novel `// $ExpectFlowError`. The error has been there the
whole time, but Flow only catches it now. Why? Who knows.

Test Plan:
Run

```
NODE_ENV=development node ./node_modules/.bin/webpack-dev-server \
    --config config/makeWebpackConfig.js
```

Note that the compilation/recompilation time is much faster than
previously.

wchargin-branch: webpack-minify-prod-only

config/makeWebpackConfig.js

@@ -21,7 +21,7 @@ const publicUrl = publicPath.slice(0, -1);
 // Get environment variables to inject into our app.
 const env = getClientEnvironment(publicUrl);
 
-function makeConfig(_unused_mode /*: "production" | "development" */) {
+function makeConfig(mode /*: "production" | "development" */) {
   return {
     // Don't attempt to continue if there are any errors.
     bail: true,
@@ -174,81 +174,7 @@ function makeConfig(_unused_mode /*: "production" | "development" */) {
         },
       ],
     },
-    plugins: [
+    plugins: plugins(mode),
-      new StaticSiteGeneratorPlugin({
-        entry: "ssr",
-        paths: require(paths.appRouteData).routeData.map(({path}) => path),
-        locals: {},
-      }),
-      // Makes some environment variables available to the JS code, for example:
-      // if (process.env.NODE_ENV === 'production') { ... }. See `./env.js`.
-      // It is absolutely essential that NODE_ENV was set to production here.
-      // Otherwise React will be compiled in the very slow development mode.
-      new webpack.DefinePlugin(env.stringified),
-      // Minify the code.
-      new webpack.optimize.UglifyJsPlugin({
-        compress: {
-          warnings: false,
-          // Disabled because of an issue with Uglify breaking seemingly valid code:
-          // https://github.com/facebookincubator/create-react-app/issues/2376
-          // Pending further investigation:
-          // https://github.com/mishoo/UglifyJS2/issues/2011
-          comparisons: false,
-        },
-        mangle: {
-          safari10: true,
-        },
-        output: {
-          comments: false,
-          // Turned on because emoji and regex is not minified properly using default
-          // https://github.com/facebookincubator/create-react-app/issues/2488
-          ascii_only: true,
-        },
-        sourceMap: shouldUseSourceMap,
-      }),
-      // Generate a manifest file which contains a mapping of all asset filenames
-      // to their corresponding output file so that tools can pick it up without
-      // having to parse `index.html`.
-      new ManifestPlugin({
-        fileName: "asset-manifest.json",
-      }),
-      // Generate a service worker script that will precache, and keep up to date,
-      // the HTML & assets that are part of the Webpack build.
-      new SWPrecacheWebpackPlugin({
-        // By default, a cache-busting query parameter is appended to requests
-        // used to populate the caches, to ensure the responses are fresh.
-        // If a URL is already hashed by Webpack, then there is no concern
-        // about it being stale, and the cache-busting can be skipped.
-        dontCacheBustUrlsMatching: /\.\w{8}\./,
-        filename: "service-worker.js",
-        logger(message) {
-          if (message.indexOf("Total precache size is") === 0) {
-            // This message occurs for every build and is a bit too noisy.
-            return;
-          }
-          if (message.indexOf("Skipping static resource") === 0) {
-            // This message obscures real errors so we ignore it.
-            // https://github.com/facebookincubator/create-react-app/issues/2612
-            return;
-          }
-          console.log(message);
-        },
-        minify: true,
-        // For unknown URLs, fallback to the index page
-        navigateFallback: publicUrl + "/index.html",
-        // Ignores URLs starting from /__ (useful for Firebase):
-        // https://github.com/facebookincubator/create-react-app/issues/2237#issuecomment-302693219
-        navigateFallbackWhitelist: [/^(?!\/__).*/],
-        // Don't precache sourcemaps (they're large) and build asset manifest:
-        staticFileGlobsIgnorePatterns: [/\.map$/, /asset-manifest\.json$/],
-      }),
-      // Moment.js is an extremely popular library that bundles large locale files
-      // by default due to how Webpack interprets its code. This is a practical
-      // solution that requires the user to opt into importing specific locales.
-      // https://github.com/jmblog/how-to-optimize-momentjs-with-webpack
-      // You can remove this if you don't use Moment.js:
-      new webpack.IgnorePlugin(/^\.\/locale$/, /moment$/),
-    ],
     // Some libraries import Node modules but don't use them in the browser.
     // Tell Webpack to provide empty mocks for them so importing them works.
     node: {
@@ -261,6 +187,95 @@ function makeConfig(_unused_mode /*: "production" | "development" */) {
   };
 }
 
+function plugins(mode /*: "development" | "production" */) {
+  const basePlugins = [
+    new StaticSiteGeneratorPlugin({
+      entry: "ssr",
+      // $ExpectFlowError(dynamic require)
+      paths: require(paths.appRouteData).routeData.map(({path}) => path),
+      locals: {},
+    }),
+    // Makes some environment variables available to the JS code, for example:
+    // if (process.env.NODE_ENV === 'production') { ... }. See `./env.js`.
+    // It is absolutely essential that NODE_ENV was set to production here.
+    // Otherwise React will be compiled in the very slow development mode.
+    new webpack.DefinePlugin(env.stringified),
+    // Generate a manifest file which contains a mapping of all asset filenames
+    // to their corresponding output file so that tools can pick it up without
+    // having to parse `index.html`.
+    new ManifestPlugin({
+      fileName: "asset-manifest.json",
+    }),
+    // Generate a service worker script that will precache, and keep up to date,
+    // the HTML & assets that are part of the Webpack build.
+    new SWPrecacheWebpackPlugin({
+      // By default, a cache-busting query parameter is appended to requests
+      // used to populate the caches, to ensure the responses are fresh.
+      // If a URL is already hashed by Webpack, then there is no concern
+      // about it being stale, and the cache-busting can be skipped.
+      dontCacheBustUrlsMatching: /\.\w{8}\./,
+      filename: "service-worker.js",
+      logger(message) {
+        if (message.indexOf("Total precache size is") === 0) {
+          // This message occurs for every build and is a bit too noisy.
+          return;
+        }
+        if (message.indexOf("Skipping static resource") === 0) {
+          // This message obscures real errors so we ignore it.
+          // https://github.com/facebookincubator/create-react-app/issues/2612
+          return;
+        }
+        console.log(message);
+      },
+      minify: true,
+      // For unknown URLs, fallback to the index page
+      navigateFallback: publicUrl + "/index.html",
+      // Ignores URLs starting from /__ (useful for Firebase):
+      // https://github.com/facebookincubator/create-react-app/issues/2237#issuecomment-302693219
+      navigateFallbackWhitelist: [/^(?!\/__).*/],
+      // Don't precache sourcemaps (they're large) and build asset manifest:
+      staticFileGlobsIgnorePatterns: [/\.map$/, /asset-manifest\.json$/],
+    }),
+    // Moment.js is an extremely popular library that bundles large locale files
+    // by default due to how Webpack interprets its code. This is a practical
+    // solution that requires the user to opt into importing specific locales.
+    // https://github.com/jmblog/how-to-optimize-momentjs-with-webpack
+    // You can remove this if you don't use Moment.js:
+    new webpack.IgnorePlugin(/^\.\/locale$/, /moment$/),
+  ];
+  const prodOnlyPlugins = [
+    // Minify the code.
+    new webpack.optimize.UglifyJsPlugin({
+      compress: {
+        warnings: false,
+        // Disabled because of an issue with Uglify breaking seemingly valid code:
+        // https://github.com/facebookincubator/create-react-app/issues/2376
+        // Pending further investigation:
+        // https://github.com/mishoo/UglifyJS2/issues/2011
+        comparisons: false,
+      },
+      mangle: {
+        safari10: true,
+      },
+      output: {
+        comments: false,
+        // Turned on because emoji and regex is not minified properly using default
+        // https://github.com/facebookincubator/create-react-app/issues/2488
+        ascii_only: true,
+      },
+      sourceMap: shouldUseSourceMap,
+    }),
+  ];
+  switch (mode) {
+    case "development":
+      return basePlugins;
+    case "production":
+      return basePlugins.concat(prodOnlyPlugins);
+    default:
+      throw new Error(/*:: (*/ mode /*: empty) */);
+  }
+}
+
 function getMode() {
   const mode = process.env.NODE_ENV;
   if (mode !== "production" && mode !== "development") {