Al Pacino, one of countless actors whose careers were nurtured by the pioneering Boston theater director David Wheeler, once described him this way to the Globe: “He’s with you like no one I ever worked with - he’s with you in the play; he’s with you in the part. He’s in the struggle with you. He’s always there, through everything.’’ Mr. Wheeler’s constancy, his vision of theater as a vitally collaborative endeavor, and his passion for championing provocative new playwrights endured to the end of his long and remarkable life, which ended Wednesday at the age of 86. He died of respiratory and heart failure, according to his son, actor and director Lewis D. Wheeler.