I have this pubspec.yaml
name: Dart Pages
description: The Dart platform.
dependencies:
web_compon
You might need to add an additional library where you import mongo_dart.dart and then import the library in your main. It's a weird solution I know but it worked for me when I was trying to get Google Maps running inside Dart.
More here: https://groups.google.com/a/dartlang.org/d/msg/misc/ORXJmmmH3fA/WIKyBik1e7sJ