Clean up violations of Dart lint "prefer_typing_uninitialized_variables". PREFER specifying a type annotation for uninitialized variables and fields.
Clean up violations of Dart lint "prefer_typing_uninitialized_variables".
PREFER specifying a type annotation for uninitialized variables and fields.

[Lint documentation](https://dart-lang.github.io/linter/lints/prefer_typing_uninitialized_variables.html)
[Dart lints discussion](http://g/dart-lints/GrWil4gpWQ0/V_0S9MjlAQAJ)

Tested:
    tap_presubmit: http://test/OCL:359424889:BASE:359405290:1614246178194:7129d106
    Sample tests for this CL passed, but some tests failed during the TGP run. Test failures are believed to be unrelated to this CL
PiperOrigin-RevId: 359562635

dart-lang/mockito