[turbofan] Assign proper type to Object.prototype.toString().

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 1487 matching lines @@
           return Type::Receiver();
         case kArrayUnshift:
           return t->cache_.kPositiveSafeInteger;
 
         // Object functions.
         case kObjectAssign:
         case kObjectCreate:
           return Type::OtherObject();
         case kObjectHasOwnProperty:
           return Type::Boolean();
+        case kObjectToString:
+          return Type::String();
 
         // RegExp functions.
         case kRegExpCompile:
           return Type::OtherObject();
         case kRegExpExec:
           return Type::Union(Type::OtherObject(), Type::Null(), t->zone());
         case kRegExpTest:
           return Type::Boolean();
         case kRegExpToString:
           return Type::String();
@@ skipping 420 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