Support the creation of Javascript libraries from Dart code

[shinayser]

This is a feature request

Today our javascript compilers (dart2js e dartdevc) doest not support the creating of standalone javascript libraries.

As the Flutter community grows every day, would be amazing if Dart could provide a way to share code created in Dart with Javascript code environment (web, node).

• dart2js requires a 'main' entry point, thus not working for libraries.
• dartdevc is not intended for production deployment and generate a huge amount of javascript code, intended for debugging.

Maybe a third compiler? Or maybe just a paramenter passed to dart2js would be enough.

In this issue I explain a little more about my failed attempt on achieving this on dart language: #46376

[Sunbreak]

Related: #35084

[mraleph]

Duplicate of #37617