Use a `package:` import when referencing source under test in test template.

This is the prefered style and plays nice w/ automated refactoring.

See: https://github.com/flutter/flutter-intellij/issues/1429

* Flutter create widget test template.

Running `flutter create —with-widget-test` produces a test/widget_test.dart sample widget test.

* Generate widget test bits in flutter create.

* Path fix.

* Added types.

* Skip shell testing on windows.

* formatting fixes

* Update test sample to test the sample app.

* Formatting tweak.