An example of different behavior of package:js for Dart2JS/DDC.

To run locally:

$ pub upgrade

# Executes with DartDevC.
$ pub run build_runner test

# Executes with Dart2JS.
$ pub run build_runner test --release

The expectation is either both or neither compilers cause the test to fail. The reality is that DDC allows and Dart2JS fails. I'm not sure which one is strictly "right".