Resolved import conflicts in example and updated to Dart 2.17.1

[rutvik110]

Hey, thanx for this awesome desktop support!🙏

I was checking out the example project and found most of the nativeshell classes having import conflicts with Flutter 3.0.1 inbuilt classes with same name. So fixed that by adding a prefix while using the nativeshell package.

Also updated example project to Dart 2.17.1