code: @Nullable
    private static Node getSubclassForEs6Superclass(Node superclass) {
      Node classNode = superclass.getParent();
      checkArgument(classNode.isClass(), classNode);
      if (NodeUtil.isNameDeclaration(classNode.getGrandparent())) {
        // const Clazz = class extends Super {
        return classNode.getParent();
      } else if (superclass.getGrandparent().isAssign()) {
        // ns.foo.Clazz = class extends Super {
        return classNode.getPrevious();
      } else if (NodeUtil.isClassDeclaration(classNode)) {
        // class Clazz extends Super {
        return classNode.getFirstChild();
      }
      return null;
    }
docs: Tries to find an lvalue for the subclass given the superclass node in an `class ... extends `
  clause
  
  <p>Only handles cases where we have either a class declaration or a class expression in an
  assignment or name declaration. Otherwise returns null.