[vm] Remove bad assert triggered when fetching heap snapshots.
[vm] Remove bad assert triggered when fetching heap snapshots.

Change-Id: Ie53edb2bf545fab80120fc268460c5c523a4a9de
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/165864
Auto-Submit: Ryan Macnak <d738b7f001baf9789095d7dc549cff6de60b011d@google.com>
Commit-Queue: Ben Konyi <67ffcaedcc3dc3d87dddb9627b72dd41756b31b1@google.com>
Reviewed-by: Ben Konyi <67ffcaedcc3dc3d87dddb9627b72dd41756b31b1@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