anchor.diff:    @observable int number;
   
     // Either SourceLocation or UnresolvedSourceLocation.
  -  @observable ServiceObject location;
  +  @observable Location location;
   
     // The breakpoint is in a file which is not yet loaded.
     @observable bool latent;
  
positive.code:     if (bpt.location.tokenPos != null) {
        line = tokenToLine(bpt.location.tokenPos);
      } else {
        line = bpt.location.line;
      }
      getLine(line).addBreakpoint(bpt);
    }
  
negative.code:     if (bpt.location.tokenPos != null) {
        line = tokenToLine(bpt.location.tokenPos);
      } else {
        UnresolvedSourceLocation loc = bpt.location;
        line = loc.line;
      }
      if (line != null) {
        getLine(line).removeBreakpoint(bpt);
  