Google requires new tests to use Mockito even if existing tests use EasyMock.
It's really hard to use two frameworks within a single test because setUp/tearDown methods invoked before and after each test perform some framework specific actions.
It's not required to convert all tests to Mockito, but the overhead of supporting both frameworks can be higher than the effort to convert existing tests.
In case of StorageImplTest, there will be many new tests developed for new Storage features, so it worths to convert these tests.