In order not to release a stable SDK where a shipped experimental flag can be disabled, we should expire the constant-update-2018 flag before D25 is released.

The flag is expired by this CL: https://dart-review.googlesource.com/c/sdk/+/113182

After the expiration is landed, we can clean up superfluous calls of the constant evaluator (this and this). These would also be useful to have in D25, as they constitute a performance overhead.