Expose Dart VM API to request multiple instances of a class, identical to what observatory is doing.

[polina-c]

[rmacnak-google]

The existing private RPC is _getInstancesAsArray. @bkonyi

[polina-c]

https://dart-review.googlesource.com/c/sdk/+/279201

[derekxu16]

FYI, the current status is that I can't land that CL yet, because it will break DWDS internally. I have a fix ready (dart-lang/webdev#1905), but the DWDS GitHub checks are currently broken.

[derekxu16]

That CL is now blocked from landing because it will break DevTools internally. DevTools must be upgraded to use package:vm_service: ^10.1.0 internally before I can land the CL.

[rmacnak-google]

The referenced CL addressed getInstances but not _getInstancesAsArray.