I am trying to create this trigger:

trigger completeFirstResponseCaseComment on CaseComment (after insert) {

// Cannot be a portal user
if (UserInfo.getUserType() == 'Standard'){
  DateTime completionDate = System.now();
  List<Id> caseIds = new List<Id>();
  for (CaseComment cc : Trigger.new){
   // Only public comments qualify
   if(cc.IsPublished == true)
    caseIds.add(cc.ParentId);
  }
  if (caseIds.isEmpty() == false){
   List<Case> caseList = [Select c.Id, c.ContactId, c.Contact.Email,
            c.OwnerId, c.Status,
            c.EntitlementId, c.SlaStartDate,
            c.SlaExitDate
            From Case c
            Where c.Id IN :caseIds];
   if (caseList.isEmpty() == false){
    List<Id> updateCases = new List<Id>();
    for (Case caseObj:caseList) {
     // consider an outbound email to the contact on the case a valid first response
     if ((caseObj.Status == 'In Progress')&&
         (caseObj.EntitlementId != null)&&
         (caseObj.SlaStartDate <= completionDate)&&
         (caseObj.SlaStartDate != null)&&
         (caseObj.SlaExitDate == null))
      updateCases.add(caseObj.Id);
    }
    if(updateCases.isEmpty() == false)
     milestoneUtils.completeMilestone(updateCases, 'First Response', completionDate);
   }
  }
}
}

Then I try to create a milestoneutils class :

public class milestoneUtils {
   
    public static void completeMilestone(List<Id> caseIds, String milestoneName, DateTime complDate) {
     
    List<CaseMilestone> cmsToUpdate = [select Id, completionDate
                       from CaseMilestone cm
                       where caseId in :caseIds and cm.MilestoneType.Name=:milestoneName and completionDate = null limit 1];
    if (cmsToUpdate.isEmpty() == false){
      for (CaseMilestone cm : cmsToUpdate){
        cm.completionDate = complDate;
      }
      update cmsToUpdate;
    } // end if
  }
 
  // test methods
  static testMethod void testCompleteMilestoneCase(){
   
    Contact oContact = [select id from Contact limit 1];
    String contactId;
    if (oContact != null)
      contactId = oContact.Id;
   
    Entitlement entl = [select id from Entitlement limit 1];
    String entlId;
    if (entl != null)
      entlId = entl.Id;
   
    List<Case> cases = new List<Case>{};
    if (entlId != null){
      Case c = new Case(Subject = 'Test Case with Entitlement ', EntitlementId = entlId, ContactId = contactId);
      cases.add(c);
    }
   
    // Insert the Account records that cause the trigger to execute.
    if (cases.isEmpty()==false){
      insert cases;
      List<Id> caseIds = new List<Id>();
      for (Case cL : cases){
        caseIds.add(cL.Id);
      }
      milestoneUtils.completeMilestone(caseIds, 'First Response', System.now());
        }
    }
 
    static testMethod void testCompleteMilestoneViaCase(){
     
        // Perform data preparation
        Entitlement entl = [select id from Entitlement limit 1];
        String entlId;
        if (entl != null)
            entlId = entl.Id;
        List<Case> cases = new List<Case>{};
        for(Integer i = 0; i < 1; i++){
            Case c = new Case(Subject = 'Test Case ' + i);
            cases.add(c);
            if (entlId != null){
                c = new Case(Subject = 'Test Case with Entitlement ' + i, EntitlementId = entlId);
                cases.add(c);
            }
        }
       
        // Insert the Account records that cause the trigger to execute.
        insert cases;

        List<CaseComment> ccs = new List<CaseComment>{};
        for(Case c : cases){
            CaseComment cc = new CaseComment(CommentBody='TestPublic', IsPublished=true, ParentId=c.Id);
            ccs.add(cc);
            cc = new CaseComment(CommentBody='TestPrivate', IsPublished=false, ParentId=c.Id);
            ccs.add(cc);
        }
        if (ccs.isEmpty()==false)
            insert ccs;
   
    // Now create emailmessage objects for them.
   
        List<EmailMessage> emails = new List<EmailMessage>();
        for(Case c : cases){
            emails.add(new EmailMessage(parentId = c.id));
        }
        if(emails.isEmpty()==false)
            database.insert(emails);
       
        for(Case c : cases){
            Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
            String[] toAddr = new String[] {'mramsey@salesforce.com'};
            mail.setToAddresses(toAddr);
            mail.setSaveAsActivity(false);
            mail.setTargetObjectId(c.ContactId);
            mail.setWhatId(c.Id);
            mail.setHtmlBody('TestHTMLBody');
            mail.setPlainTextBody('TestTextBody');
            Messaging.sendEmail(new Messaging.SingleEmailMessage[] { mail });
        }
   
    for(Case c : cases){
      c.Status = 'Closed';
    }
    update cases;
   
        // Query the database for the newly inserted records.
        List<Case> insertedCases = [SELECT Subject,
                                           Description,
                                          (SELECT IsPublished, CommentBody From CaseComments),
                                          (SELECT TextBody, Subject, Incoming From EmailMessages)
                                           FROM Case
                                           WHERE Id IN :cases];
    }
}

But still not luck, I get this error:
: Compile Error: Didn't understand relationship 'EmailMessages' in FROM part of query call. If you are attempting to use a custom relationship, be sure to append the '__r' after the custom relationship name. Please reference your WSDL or the describe call for the appropriate names. at line 103 column 36