Don't use webpack multi entry unnecessarily (facebook#8834)

Passing an array with a single entry is not equivalent. This causes Webpack to generate another wrapper module around the entry. This is just unnecessary overhead and bytes.

Author: sebmarkbage

packages/react-scripts/config/webpack.config.js

@@ -157,7 +157,7 @@ module.exports = function(webpackEnv) {
  : isEnvDevelopment && 'cheap-module-source-map',
  // These are the "entry points" to our application.
  // This means they will be the "root" imports that are included in JS bundle.
- entry: [
+ entry: isEnvDevelopment && !shouldUseReactRefresh ? [
  // Include an alternative client for WebpackDevServer. A client's job is to
  // connect to WebpackDevServer by a socket and get notified about changes.
  // When you save a file, the client will either apply hot updates (in case
@@ -172,13 +172,13 @@ module.exports = function(webpackEnv) {
  //
  // When using the experimental react-refresh integration,
  // the webpack plugin takes care of injecting the dev client for us.
- isEnvDevelopment && !shouldUseReactRefresh && webpackDevClientEntry,
+ webpackDevClientEntry,
  // Finally, this is your app's code:
  paths.appIndexJs,
  // We include the app code last so that if there is a runtime error during
  // initialization, it doesn't blow up the WebpackDevServer client, and
  // changing JS code would still trigger a refresh.
- ].filter(Boolean),
+ ] : paths.appIndexJs,
  output: {
  // The build folder.
  path: isEnvProduction ? paths.appBuild : undefined,