Introduce extended mode.

src/ast.h

Index: src/ast.h
diff --git a/src/ast.h b/src/ast.h
index 295257aaebdeb4a0b716af0f604930062d8c80d5..bb032cac0e938ac0a33ff02808021847b5d75878 100644
--- a/src/ast.h
+++ b/src/ast.h
@@ -1631,8 +1631,8 @@ class FunctionLiteral: public Expression {
   int end_position() const;
   bool is_expression() const { return is_expression_; }
   bool is_anonymous() const { return is_anonymous_; }
-  bool strict_mode() const { return strict_mode_flag() == kStrictMode; }
-  StrictModeFlag strict_mode_flag() const;
+  bool is_classic_mode() const { return language_mode() == CLASSIC_MODE; }
+  LanguageMode language_mode() const;
 
   int materialized_literal_count() { return materialized_literal_count_; }
   int expected_property_count() { return expected_property_count_; }