Correctly specify react and react-dom as externals

Previously, the syntax was incorrect and was not doing anything.
Additionally, it was attempting to set them as externals in the playground
builds, which is incorrect, they need to be included in the playground build.

webpack.config.js

@@ -27,10 +27,6 @@ const base = {
         filename: '[name].js',
         chunkFilename: 'chunks/[name].js'
     },
-    externals: {
-        React: 'react',
-        ReactDOM: 'react-dom'
-    },
     resolve: {
         symlinks: false
     },
@@ -113,10 +109,6 @@ module.exports = [
             path: path.resolve(__dirname, 'build'),
             filename: '[name].js'
         },
-        externals: {
-            React: 'react',
-            ReactDOM: 'react-dom'
-        },
         module: {
             rules: base.module.rules.concat([
                 {
@@ -200,8 +192,8 @@ module.exports = [
                 publicPath: `${STATIC_PATH}/`
             },
             externals: {
-                React: 'react',
-                ReactDOM: 'react-dom'
+                'react': 'react',
+                'react-dom': 'react-dom'
             },
             module: {
                 rules: base.module.rules.concat([