code: private void init(Symtab syms, boolean definitive) {
          if (classes != null) return;
  
          if (definitive) {
              Assert.check(packages == null || packages == syms.packages);
              packages = syms.packages;
              Assert.check(classes == null || classes == syms.classes);
              classes = syms.classes;
          } else {
              packages = new HashMap<Name, PackageSymbol>();
              classes = new HashMap<Name, ClassSymbol>();
          }
  
          packages.put(names.empty, syms.rootPackage);
          syms.rootPackage.completer = thisCompleter;
          syms.unnamedPackage.completer = thisCompleter;
      }
docs: Initialize classes and packages, optionally treating this as
  the definitive classreader.