code: function classDeclarationExtendsNull(classDecl) {
              var classSymbol = getSymbolOfNode(classDecl);
              var classInstanceType = getDeclaredTypeOfSymbol(classSymbol);
              var baseConstructorType = getBaseConstructorTypeOfClass(classInstanceType);
              return baseConstructorType === nullWideningType;
          }
docs: Check if the given class-declaration extends null then return true.
  Otherwise, return false
  @param classDecl a class declaration to check if it extends null