Try removing an extra null check in cloneNullableToken.
Try removing an extra null check in cloneNullableToken.

When trying to update to analyzer 1.3.0 dartdoc started crashing
occasionally in CI with the following exception:

https://github.com/dart-lang/dartdoc/pull/2589/checks?check_run_id=2195215917#step:6:3411

Reading the code suggests that returning a null here should be an
OK thing to do, but if this is intended to enforce that an invalid
key should never be passed to this method, some other solution
will need to be found.

Change-Id: Id210a0f8feb6197e7a65569bfaed4d599be4b3c7
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/193062
Reviewed-by: Konstantin Shcheglov <09e4d7516628963212bf4aace2f97603d2b706e4@google.com>
Commit-Queue: Janice Collins <f6a8c838e29d3fa8813240c8bed106e760081e0e@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk