We should verify that element type is nullable every time length is
increased, not only when capacity is increased.
Fixes https://github.com/dart-lang/sdk/issues/42502
Change-Id: Id91c702a99028634da8c2d41ae0ceac521af2cf5
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/152900
Reviewed-by: Régis Crelier <regis@google.com>
Reviewed-by: Lasse R.H. Nielsen <lrn@google.com>
Commit-Queue: Alexander Markov <alexmarkov@google.com>
