// Microsoft.SharePoint.WorkflowServices.StoreSubscriptionService
public override WorkflowSubscriptionCollection EnumerateSubscriptionsByEventSource(Guid eventSourceId)
{
    this.context.Web.CheckPermissions(SPBasePermissions.EditListItems);
    WorkflowStore workflowStore = new WorkflowStore(this.context.Web);
    eventSourceId = StoreSubscriptionService.ConvertToGuidToken(eventSourceId, this.context.Web);
    WorkflowFile[] files = workflowStore.QueryWithGuid("0x0100AA27A923036E459D9EF0D18BBD0B9587", StoragePublishState.Unchanged, "WSEventSourceGUID", eventSourceId);
    return this.ConvertToWorkflowSubscriptionCollection(files);
}