code: Tracer newTracer(String passName) {
      String comment = passName
          + (recentChange.hasCodeChanged() ? " on recently changed AST" : "");
      if (options.getTracerMode().isOn() && tracker != null) {
        tracker.recordPassStart(passName, true);
      }
      return new Tracer("Compiler", comment);
    }
docs: Returns a new tracer for the given pass name.