You need to create an analysis_options.yaml file in the root of your flutter app and write something like
analyzer:
enable-experiment:
- spread-collections
Also make sure to switch to the correct channel where the new feature is included eg (dev, beta or master)
flutter channel dev
And also make sure you have a recent enough version of flutter
flutter upgrade
Ensure you are on the right version of flutter and dart that allows that feature by running
flutter --version
you may also have to manually change your pubspec.yaml file to specify the correct dart sdk (if so rerun flutter upgrade)
environment:
sdk: ">=2.10.0-0 <3.0.0"