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,
@@ -174,9 +174,24 @@ function makeConfig(_unused_mode /*: "production" | "development" */) {
         },
       ],
     },
-    plugins: [
+    plugins: plugins(mode),
+    // 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: {
+      dgram: "empty",
+      fs: "empty",
+      net: "empty",
+      tls: "empty",
+      child_process: "empty",
+    },
+  };
+}
+
+function plugins(mode /*: "development" | "production" */) {
+  const basePlugins = [
     new StaticSiteGeneratorPlugin({
       entry: "ssr",
+      // $ExpectFlowError(dynamic require)
       paths: require(paths.appRouteData).routeData.map(({path}) => path),
       locals: {},
     }),
@@ -185,27 +200,6 @@ function makeConfig(_unused_mode /*: "production" | "development" */) {
     // It is absolutely essential that NODE_ENV was set to production here.
     // Otherwise React will be compiled in the very slow development mode.
     new webpack.DefinePlugin(env.stringified),
-      // Minify the code.
-      new webpack.optimize.UglifyJsPlugin({
-        compress: {
-          warnings: false,
-          // Disabled because of an issue with Uglify breaking seemingly valid code:
-          // https://github.com/facebookincubator/create-react-app/issues/2376
-          // Pending further investigation:
-          // https://github.com/mishoo/UglifyJS2/issues/2011
-          comparisons: false,
-        },
-        mangle: {
-          safari10: true,
-        },
-        output: {
-          comments: false,
-          // Turned on because emoji and regex is not minified properly using default
-          // https://github.com/facebookincubator/create-react-app/issues/2488
-          ascii_only: true,
-        },
-        sourceMap: shouldUseSourceMap,
-      }),
     // Generate a manifest file which contains a mapping of all asset filenames
     // to their corresponding output file so that tools can pick it up without
     // having to parse `index.html`.
@@ -248,17 +242,38 @@ function makeConfig(_unused_mode /*: "production" | "development" */) {
     // https://github.com/jmblog/how-to-optimize-momentjs-with-webpack
     // You can remove this if you don't use Moment.js:
     new webpack.IgnorePlugin(/^\.\/locale$/, /moment$/),
-    ],
+  ];
-    // Some libraries import Node modules but don't use them in the browser.
+  const prodOnlyPlugins = [
-    // Tell Webpack to provide empty mocks for them so importing them works.
+    // Minify the code.
-    node: {
+    new webpack.optimize.UglifyJsPlugin({
-      dgram: "empty",
+      compress: {
-      fs: "empty",
+        warnings: false,
-      net: "empty",
+        // Disabled because of an issue with Uglify breaking seemingly valid code:
-      tls: "empty",
+        // https://github.com/facebookincubator/create-react-app/issues/2376
-      child_process: "empty",
+        // 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() {