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