On the web, the size of the JavaScript version of your app matters. For this reason, dart2js is optimized to produce the smallest possible JavaScript files.
When you're ready to deploy, minify the size of the generated JavaScript with –m or -- minify, as shown in the following command:
dart2js –m -o prorabbits.js prorabbits.dart
Using pub build on the command line minifies JavaScript by default because this command is meant for deployment.
The dart2js file utilizes a tree-shaking feature; only code that is necessary during execution is retained, that is, functions, classes, and libraries that are not called are excluded from the produced .js file. The minification process further reduces the size by replacing the names of variables, functions, and so on with shorter names and moving code around to use a few lines.
Be careful when you use reflection.
Using reflection in the Dart code prevents tree shaking. So only import the dart:mirrors library when you really have to. In this case, include an @MirrorsUsed annotation, as shown in the following code:
library mylib; @MirrorsUsed(targets: 'mylib') import 'dart:mirrors';
In the previous code, all the names and entities (classes, functions, and so on) inside of mylib will be retained in the generated code to use reflection. So create a separate library to hold the class that is using mirrors.
You might want to consult the Using Reflection recipe in Chapter 4, Object Orientation.