[gardening] Fix transferable_test - remove timing assumptions.
[gardening] Fix transferable_test - remove timing assumptions.

Attempts to expect runtime performances lead to flakiness.

Fixes dartbug.com/37520

Change-Id: I6bc548f523b2c2453c7fa90498ebaf4f91ec9f71
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/147380
Reviewed-by: Alexander Markov <239e774fd29abc1267878f57afe1a8a03e0ca887@google.com>
Commit-Queue: Alexander Aprelev <dfb74c35a357c2059971a4bf7fe26b6a7c18085d@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