[vm/doc] Add examples to `WeakReference` and `NativeFinalizer` take 2
[vm/doc] Add examples to `WeakReference` and `NativeFinalizer` take 2

Change-Id: Iabafdcc45052bf76f4d13e32566149efd8f71adf
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/243263
Commit-Queue: Michael Thomsen <5977a55c75bb4612004451a56b0b4c22d9d3311d@google.com>
Commit-Queue: Daco Harkes <930cbae0ca77f1954c53e32620d89cfcca318f00@google.com>
Reviewed-by: Michael Thomsen <5977a55c75bb4612004451a56b0b4c22d9d3311d@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