code: public ImmutableSet<ClassInfo> getTopLevelClasses(String packageName) {
      checkNotNull(packageName);
      ImmutableSet.Builder<ClassInfo> builder = ImmutableSet.builder();
      for (ClassInfo classInfo : getTopLevelClasses()) {
        if (classInfo.getPackageName().equals(packageName)) {
          builder.add(classInfo);
        }
      }
      return builder.build();
    }
docs: Returns all top level classes whose package name is {@code packageName}.