[turbofan] Implement JSBuiltinReducer for String.prototype[Symbol.iterator].

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 1332 matching lines @@
           return Type::Union(Type::Range(0, kMaxUInt16, t->zone()), Type::NaN(),
                              t->zone());
         case kStringCharAt:
         case kStringConcat:
         case kStringFromCharCode:
         case kStringSubstr:
         case kStringToLowerCase:
         case kStringToUpperCase:
           return Type::String();
 
+        case kStringIterator:
         case kStringIteratorNext:
           return Type::OtherObject();
 
         // Array functions.
         case kArrayIndexOf:
         case kArrayLastIndexOf:
           return Type::Range(-1, kMaxSafeInteger, t->zone());
         // Object functions.
         case kObjectHasOwnProperty:
           return Type::Boolean();
@@ skipping 340 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