[observatory] Remove stale asserts from heap snapshot processing.
[observatory] Remove stale asserts from heap snapshot processing.

The new heap snapshot format includes VM isolate objects as objects with size 0 instead of excluding them.

Bug: https://github.com/dart-lang/sdk/issues/37919
Change-Id: I5bfd2ca1982e4d7a184907b8981838bdc280d8d2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/113751
Reviewed-by: Alexander Markov <239e774fd29abc1267878f57afe1a8a03e0ca887@google.com>
Commit-Queue: Ryan Macnak <d738b7f001baf9789095d7dc549cff6de60b011d@google.com>

dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk