@bkonyi chrome proxy service throws a sentinel exception on getIsolate for non-existing isolate id, as required by the VmServiceInterface, but the VmServerConnection translates it into an RPC error with RPCError.kInternalError as code, and the sentinel is added to the message as a string, so I have to check for the string contents here. Is there a better way to handle this error? Should we change the VMServer connection to throw something different, or have a set of error codes in package:vm_service we all agree on and can check for here?

service.getIsolate(...) should be throwing a SentinelException in this case, so there's likely something wrong with the encoding on the DWDS side of things. What's the JSON response you're seeing here? It should look something like:

{
  "id": "123",
  "result": {
    "type": "Sentinel",
    "kind": "Collected",
    "valueAsString": "..."
  }
}

If I had to guess, the "error" property is being populated instead, which is incorrect.

dwds's Chrome proxy service throws a sentinel exception, but the vm server connection we use on top of it converts it to RPC error with code of RPCError.kInternalError here:
https://github.com/dart-lang/sdk/blob/67f3e1f4a0dfe57c07192607095892a9d107a9d1/pkg/vm_service/lib/src/vm_service.dart#L1795

Should this code be changed then?

I will file an issue in the SDK repo for this, in the meanwhile the current PR should fix the flake.

Filed an issue (i am worried about all the other cases where we miss error checks due to this problem): dart-lang/sdk#51752