code: private Set<Source> calculateModifiedSources() {
          Set<Source> modified = new HashSet<>();
          for (String src : now.sources().keySet()) {
              Source n = now.sources().get(src);
              Source t = prev.sources().get(src);
              if (prev.sources().get(src) != null) {
                  if (t != null) {
                      if (n.lastModified() > t.lastModified()) {
                          modified.add(n);
                      } else if (n.lastModified() < t.lastModified()) {
                          modified.add(n);
                          Log.warn("The source file "+n.name()+" timestamp has moved backwards in time.");
                      }
                  }
              }
          }
          return modified;
      }
docs: Return those files where the timestamp is newer.
  If a source file timestamp suddenly is older than what is known
  about it in javac_state, then consider it modified, but print
  a warning!