Implement Q1 2021 features. Add/update co19 tests for "Allow generic function types as type arguments"

Add/update co19 tests for "[Allow generic function types as type arguments](https://github.com/dart-lang/language/issues/496)" feature. 
[Specification](https://github.com/dart-lang/language/blob/master/accepted/future-releases/small-features-21Q1/feature-specification.md#allow-generic-function-types-as-type-arguments) 

Meta-issue https://github.com/dart-lang/sdk/issues/44911