Shared_Preferences for Flutter Web?

shared_preferences are not supposed to work with flutter web, that's why value of instance never returns. For this purpose, you can use any key-value stores instead, for example, sembast

UPD: the package supports web now since version 0.5.6

Great news, from version 0.5.6 shared_prefs flutter supports web by default

Now it's includes shared_preferences for web

Your code should work without changes, just update dependency in pubspec.yaml

dependencies:
 shared_preferences: ^0.5.6