Add a dev-only config for webpack and optimize main build for production

Summary:
This diff optimizes our `webpack.config.js` for production by pointing to the React production build
(to disable `PropType` validation, which is quite slow), enabling Uglify, and disabling source maps.
In addition, it defines a separate `webpack.config.dev.js` which reverts all of these changes and an
`npm run watch` command that points to the dev settings. This is a similar setup to what we have for
our mobile JS (see [here](https://github.com/Khan/mobile-client-webview-resources/tree/master/javascript)).

On Asana @ https://app.asana.com/0/114493820703878/119470924649091.

Test Plan:
- Run `webpack`.
    - Verify that the produced build is relatively small (< 400 KB).
    - Open up the keypad.
    - Enable profiling in Devtools.
    - Swipe around a bit.
        - Verify that there is no `PropType` validation going on.
- Run `npm run watch`.
    - Verify that the produced build is much larger (~2 MB).
    - Verify that `PropType` validation is back.

Reviewers: kevinb

Reviewed By: kevinb

Differential Revision: https://phabricator.khanacademy.org/D27229

package.json

@@ -6,7 +6,7 @@
   "scripts": {
     "test": "mocha test/test_*.js --compilers js:babel-register",
     "watch_test": "mocha test/test_spec.js --watch --compilers js:babel-register",
-    "start": "webpack -w"
+    "watch": "webpack --config webpack.config.dev.js --watch"
   },
   "author": "",
   "license": "MIT",

webpack.config.dev.js

@@ -0,0 +1,17 @@
+const webpack = require('webpack');
+
+const config = require('./webpack.config');
+
+// Same plugins as the production config, except:
+//  (1) No Uglify minification.
+//  (2) No production Node environment (which is used in production to disable
+//      React's PropTypes validation).
+config.plugins = [
+    new webpack.optimize.CommonsChunkPlugin({
+        name: 'deps',
+    }),
+];
+
+config.devtool = '#eval-source-map';
+
+module.exports = config;

webpack.config.js

@@ -2,7 +2,6 @@ const path = require('path');
 const webpack = require('webpack');
 
 module.exports = {
-    devtool: '#eval-source-map',
     entry: {
         app: './src/index',
         deps: [
@@ -23,11 +22,14 @@ module.exports = {
         new webpack.optimize.CommonsChunkPlugin({
             name: 'deps',
         }),
-        // new webpack.optimize.UglifyJsPlugin({
-        //     compress: {
-        //         warnings: false
-        //     }
-        // }),
+        new webpack.DefinePlugin({
+            'process.env.NODE_ENV': '"production"',
+        }),
+        new webpack.optimize.UglifyJsPlugin({
+            compress: {
+                warnings: false,
+            },
+        }),
     ],
     resolve: {
         extensions: ['', '.js', '.jsx'],