public class MilestoneSwimlaneController{
    private final ID opp;
    public MilestoneSwimlaneController(ApexPages.StandardController oppcontroller) {
       opp =  ApexPages.currentPage().getParameters().get('id');
    }
    List<Milestone__c> miles = [Select id from milestone__c where Opportunity_milestones__c = :opp];
    Public List<Milestone__c> getmiles(){
        return miles;
    }
}