Ensure scopes are backed by blocks in the body of for loops

src/parser.cc

Index: src/parser.cc
diff --git a/src/parser.cc b/src/parser.cc
index f122d9aa5e4b57ff1459cfa48e060c666bd407ff..04ce30f4f383733d32efe87786838ade2e3d0431 100644
--- a/src/parser.cc
+++ b/src/parser.cc
@@ -3786,12 +3786,19 @@ Statement* Parser::ParseForStatement(ZoneList<const AstRawString*>* labels,
         Expression* enumerable = ParseExpression(true, CHECK_OK);
         Expect(Token::RPAREN, CHECK_OK);
 
+        // Make a block around the statement in case a lexical binding
+        // is introduced, e.g. by a FunctionDeclaration.
+        Block* block =
+            factory()->NewBlock(NULL, 1, false, RelocInfo::kNoPosition);
         Statement* body = ParseSubStatement(NULL, CHECK_OK);
-        InitializeForEachStatement(loop, expression, enumerable, body);
+        block->statements()->Add(body, zone());
+        InitializeForEachStatement(loop, expression, enumerable, block);
         scope_ = saved_scope;
         for_scope->set_end_position(scanner()->location().end_pos);
         for_scope = for_scope->FinalizeBlockScope();
-        DCHECK(for_scope == NULL);
+        if (for_scope != nullptr) {
+          block->set_scope(for_scope);
+        }
         // Parsed for-in loop.
         return loop;
 
@@ -3861,10 +3868,20 @@ Statement* Parser::ParseForStatement(ZoneList<const AstRawString*>* labels,
       //     const x = i;
       //     for (; c; n) b
       //   }
-      DCHECK(init != NULL);
+      //
+      // or, desugar
+      //   for (; c; n) b
+      // into
+      //   {
+      //     for (; c; n) b
+      //   }
+      // just in case b introduces a lexical binding some other way, e.g., if b
+      // is a FunctionDeclaration.
       Block* block =
           factory()->NewBlock(NULL, 2, false, RelocInfo::kNoPosition);
-      block->statements()->Add(init, zone());
+      if (init != nullptr) {
+        block->statements()->Add(init, zone());
+      }
       block->statements()->Add(loop, zone());
       block->set_scope(for_scope);
       loop->Initialize(NULL, cond, next, body);