Add strong mode.

src/preparser.h

Index: src/preparser.h
diff --git a/src/preparser.h b/src/preparser.h
index be27be75f75a66ad6b21590808467e91b081d210..99b3fe5fc96f7f8a37cc92c1de33cdd9c2a4d198 100644
--- a/src/preparser.h
+++ b/src/preparser.h
@@ -754,8 +754,7 @@ class PreParserExpression {
   }
 
   static PreParserExpression StringLiteral() {
-    return PreParserExpression(TypeField::encode(kStringLiteralExpression) |
-                               IsUseStrictField::encode(false));
+    return PreParserExpression(TypeField::encode(kStringLiteralExpression));
   }
 
   static PreParserExpression UseStrictStringLiteral() {
@@ -763,6 +762,11 @@ class PreParserExpression {
                                IsUseStrictField::encode(true));
   }
 
+  static PreParserExpression UseSanityStringLiteral() {
+    return PreParserExpression(TypeField::encode(kStringLiteralExpression) |
+                               IsUseSanityField::encode(true));
+  }
+
   static PreParserExpression This() {
     return PreParserExpression(TypeField::encode(kExpression) |
                                ExpressionTypeField::encode(kThisExpression));
@@ -814,6 +818,11 @@ class PreParserExpression {
            IsUseStrictField::decode(code_);
   }
 
+  bool IsUseSanityLiteral() const {
+    return TypeField::decode(code_) == kStringLiteralExpression &&
+           IsUseSanityField::decode(code_);
+  }
+
   bool IsThis() const {
     return TypeField::decode(code_) == kExpression &&
            ExpressionTypeField::decode(code_) == kThisExpression;
@@ -922,6 +931,7 @@ class PreParserExpression {
   typedef BitField<ExpressionType, ParenthesizationField::kNext, 3>
       ExpressionTypeField;
   typedef BitField<bool, ParenthesizationField::kNext, 1> IsUseStrictField;
+  typedef BitField<bool, IsUseStrictField::kNext, 1> IsUseSanityField;
   typedef BitField<bool, ParenthesizationField::kNext, 1>
       IsValidArrowParamListField;
   typedef BitField<PreParserIdentifier::Type, ParenthesizationField::kNext, 10>
@@ -963,6 +973,9 @@ class PreParserStatement {
     if (expression.IsUseStrictLiteral()) {
       return PreParserStatement(kUseStrictExpressionStatement);
     }
+    if (expression.IsUseSanityLiteral()) {
+      return PreParserStatement(kUseSanityExpressionStatement);
+    }
     if (expression.IsStringLiteral()) {
       return PreParserStatement(kStringLiteralExpressionStatement);
     }
@@ -977,6 +990,8 @@ class PreParserStatement {
     return code_ == kUseStrictExpressionStatement;
   }
 
+  bool IsUseSanityLiteral() { return code_ == kUseSanityExpressionStatement; }
+
   bool IsFunctionDeclaration() {
     return code_ == kFunctionDeclaration;
   }
@@ -986,6 +1001,7 @@ class PreParserStatement {
     kUnknownStatement,
     kStringLiteralExpressionStatement,
     kUseStrictExpressionStatement,
+    kUseSanityExpressionStatement,
     kFunctionDeclaration
   };