If you go this route, your call sites that are passing
tester.binding.clock won’t need to be modified,
but other call sites will.

Option #3: Change the API to accept a DateTime function()

If you only use the Clock for its now function,
and you control the API, then you can also change it
to accept that function directly instead of a Clock.
This makes it easily callable with either type of Clock,
by passing a tearoff of the now method from either type of clock:

Calling code before migration:

testWidgets

'some test'

WidgetTester

tester

someApiThatWantsAQuiverClock

tester

binding

clock

);

});

Calling code after migration:

testWidgets

'some test'