Stop emitting kSloppyLexical errors when --harmony-sloppy-let is enabled

This changes the error message for code like:

  if (false) let x;

from "Block-scoped declarations (let, const, function, class) not yet supported outside strict mode"
to "Unexpected identifier" (pointing at |x|).

Review URL: https://codereview.chromium.org/1356783002

Cr-Commit-Position: refs/heads/master@{#30834}

src/parser.cc

@@ -2713,7 +2713,8 @@ Statement* Parser::ParseExpressionOrLabelledStatement(
 
   // Parsed expression statement, followed by semicolon.
   // Detect attempts at 'let' declarations in sloppy mode.
-  if (peek() == Token::IDENTIFIER && expr->AsVariableProxy() != NULL &&
+  if (!allow_harmony_sloppy_let() && peek() == Token::IDENTIFIER &&
+      expr->AsVariableProxy() != NULL &&
       expr->AsVariableProxy()->raw_name() ==
           ast_value_factory()->let_string()) {
     ReportMessage(MessageTemplate::kSloppyLexical, NULL);
@@ -3792,8 +3793,8 @@ Statement* Parser::ParseForStatement(ZoneList<const AstRawString*>* labels,
 
   // Parsed initializer at this point.
   // Detect attempts at 'let' declarations in sloppy mode.
-  if (peek() == Token::IDENTIFIER && is_sloppy(language_mode()) &&
-      is_let_identifier_expression) {
+  if (!allow_harmony_sloppy_let() && peek() == Token::IDENTIFIER &&
+      is_sloppy(language_mode()) && is_let_identifier_expression) {
     ReportMessage(MessageTemplate::kSloppyLexical, NULL);
     *ok = false;
     return NULL;

src/preparser.cc

@@ -700,8 +700,9 @@ PreParser::Statement PreParser::ParseExpressionOrLabelledStatement(bool* ok) {
   }
   // Parsed expression statement.
   // Detect attempts at 'let' declarations in sloppy mode.
-  if (peek() == Token::IDENTIFIER && is_sloppy(language_mode()) &&
-      expr.IsIdentifier() && expr.AsIdentifier().IsLet()) {
+  if (!allow_harmony_sloppy_let() && peek() == Token::IDENTIFIER &&
+      is_sloppy(language_mode()) && expr.IsIdentifier() &&
+      expr.AsIdentifier().IsLet()) {
     ReportMessage(MessageTemplate::kSloppyLexical, NULL);
     *ok = false;
     return Statement::Default();
@@ -960,8 +961,8 @@ PreParser::Statement PreParser::ParseForStatement(bool* ok) {
 
   // Parsed initializer at this point.
   // Detect attempts at 'let' declarations in sloppy mode.
-  if (peek() == Token::IDENTIFIER && is_sloppy(language_mode()) &&
-      is_let_identifier_expression) {
+  if (!allow_harmony_sloppy_let() && peek() == Token::IDENTIFIER &&
+      is_sloppy(language_mode()) && is_let_identifier_expression) {
     ReportMessage(MessageTemplate::kSloppyLexical, NULL);
     *ok = false;
     return Statement::Default();