Add support for `mocktail` package

[mousedownmike]

Please add support for the mocktail package.

Mock library for Dart inspired by mockito.

Mocktail focuses on providing a familiar, simple API for creating mocks in Dart (with null-safety) without the need for manual mocks or code generation.

Having DartPad access would assist with creating interactive documentation for the project.

[johnpryan]

This depends on the test package, which DartPad doesn't support right now.

[mousedownmike]

Thanks for the update, relates to #1348