[turbofan] for-in only enumerates Strings.

src/compiler/typer.cc

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/compiler/typer.h"
 
 #include <iomanip>
 
 #include "src/base/flags.h"
 #include "src/bootstrapper.h"
@@ skipping 1623 matching lines @@
   return Type::Any();
 }
 
 
 Type* Typer::Visitor::TypeJSConvertReceiver(Node* node) {
   return Type::Receiver();
 }
 
 
 Type* Typer::Visitor::TypeJSForInNext(Node* node) {
-  return Type::Union(Type::Name(), Type::Undefined(), zone());
+  return Type::Union(Type::String(), Type::Undefined(), zone());
 }
 
 
 Type* Typer::Visitor::TypeJSForInPrepare(Node* node) {
   STATIC_ASSERT(Map::EnumLengthBits::kMax <= FixedArray::kMaxLength);
   Type* const cache_type =
       Type::Union(Type::SignedSmall(), Type::OtherInternal(), zone());
   Type* const cache_array = Type::OtherInternal();
   Type* const cache_length = typer_->cache_.kFixedArrayLengthType;
   return Type::Tuple(cache_type, cache_array, cache_length, zone());
@@ skipping 291 matching lines @@
 Type* Typer::Visitor::TypeConstant(Handle<Object> value) {
   if (Type::IsInteger(*value)) {
     return Type::Range(value->Number(), value->Number(), zone());
   }
   return Type::NewConstant(value, zone());
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8