import 'package:mongo_dart/mongo_dart.dart';

void main() async {

Db db = new Db("mongodb://localhost:27017/test");
await db.open();
print('Connection succeeded!');
//the test database is accessible from Compass or terminal
//localhost:27017 works well
}

await db.open() throws that error:
dart_sdk.js:5822 Uncaught Error: Unsupported operation: Socket
constructor
at Object.dart.throw (dart_sdk.js:4537)
at Function._connect (dart_sdk.js:52279)
at Function.connect (dart_sdk.js:52264)
at mongo_dart._Connection.new.connect (connection.dart:56)
at mongo_dart._ConnectionManager.new._connect
(connection_manager.dart:23)
at _connect.next (<anonymous>)
at runBody (dart_sdk.js:22269)
at Object.async.async (dart_sdk.js:22297)
at mongo_dart._ConnectionManager.new.[_connect]
(connection_manager.dart:22)
at connection_manager.dart:58
at dart_sdk.js:23887