Introduce LanguageMode, drop StrictMode.

src/token.h

Index: src/token.h
diff --git a/src/token.h b/src/token.h
index 9fc330acddb8db1f32331c3265ef0b8a4903a486..2c2ad84e5722f91da525732a11393df20d72f0f8 100644
--- a/src/token.h
+++ b/src/token.h
@@ -193,7 +193,7 @@ class Token {
     return token_type[tok] == 'K';
   }
 
-  static bool IsIdentifier(Value tok, StrictMode strict_mode,
+  static bool IsIdentifier(Value tok, LanguageMode language_mode,
                            bool is_generator) {
     switch (tok) {
       case IDENTIFIER:
@@ -201,9 +201,9 @@ class Token {
       case FUTURE_STRICT_RESERVED_WORD:
       case LET:
       case STATIC:
-        return strict_mode == SLOPPY;
+        return !is_strict(language_mode);
       case YIELD:
-        return !is_generator && strict_mode == SLOPPY;
+        return !is_generator && !is_strict(language_mode);
       default:
         return false;
     }