Remove use of "implements Function" in tests.

[ferhatb]

Description

implements/extends Function is being deprecated. This PR prepares flutter code base.

Related Issues

Example Dart roll failure: https://cirrus-ci.com/task/4670267658076160

Tests

Already covered by icon_button_test and implicit_animations_test.

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[ditman]

LGTM

[ditman]

This looks like something that could be achieved with some Mockito mocks, instead of rolling our own function-like handler?

[yjbanov]

@vsmenon @sigmundch Surprisingly this only failed on the Web, not on mobile. Is there a discrepancy between Web and VM we should be aware of?

[vsmenon]

@yjbanov - web and mobile should behave the same here. Are these files exercised in both places? Do both have consistent settings wrt failing on warnings?

fyi - @johnniwinther

[yjbanov]

Are these files exercised in both places?

Yes.

Do both have consistent settings wrt failing on warnings?

/cc @jonahwilliams do you know?