fix(@angular/cli): in ng serve prefix historyApiFallback.index with deployUrl

when implementing i18n following https://angular.io/docs/ts/latest/cookbook/i18n.html
and https://medium.com/@feloy/deploying-an-i18n-angular-app-with-angular-cli-fc788f17e358

when running in `development` to be consistent with `production` I am using

```
ng serve --deploy-url /en/ --base-href /en/ \
--aot --locale en --i18n-file ./src/locale/messages.en.xlf
```

running the app this way, on routes other than `http://localhost:4200/en/`
the `webpack-dev-server` was always saying that the route is not found,
so this commit fixes that

Author: smoke

packages/@angular/cli/tasks/serve.ts

@@ -177,6 +177,8 @@ export default Task.extend({
     // set publicPath property to be sent on webpack server config
     if (serveTaskOptions.deployUrl) {
       webpackDevServerConfiguration.publicPath = serveTaskOptions.deployUrl;
+      (webpackDevServerConfiguration.historyApiFallback as any).index =
+        serveTaskOptions.deployUrl + `/${appConfig.index}`;
     }
 
     if (serveTaskOptions.target === 'production') {