[dart2js] Deferred loading - don't leak ImportSet(s)
[dart2js] Deferred loading - don't leak ImportSet(s)

Null-out DeferredLoadTask.importSets to free up all the ImportSets.

DeferredLoadTask
  -> ImportSetLattice
    -> ImportSet({})
      -> all other ImportSets via transitions.

Change-Id: I552b4a37396277b9a43a6a823622ec2d7a542848
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/148226
Reviewed-by: Sigmund Cherem <beaea8fd7d2a56555046ba4f362c8a93e3938576@google.com>
Commit-Queue: Stephen Adams <60cb5b73b7e0cb7dcda53302da4fab6ecfc7fa8b@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