Remove flags for ES2015 features shipped in M48

src/js/symbol.js

 // Copyright 2013 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.
 
 (function(global, utils) {
 
 "use strict";
 
 %CheckIsBootstrapping();
 
@@ skipping 59 matching lines @@
 function SymbolKeyFor(symbol) {
   if (!IS_SYMBOL(symbol)) throw MakeTypeError(kSymbolKeyFor, symbol);
   return %SymbolRegistry().keyFor[symbol];
 }
 
 // -------------------------------------------------------------------
 
 utils.InstallConstants(GlobalSymbol, [
   // TODO(rossberg): expose when implemented.
   // "hasInstance", hasInstanceSymbol,
-  // "isConcatSpreadable", isConcatSpreadableSymbol,
+  "isConcatSpreadable", isConcatSpreadableSymbol,
   "iterator", iteratorSymbol,
   // TODO(yangguo): expose when implemented.
   // "match", matchSymbol,
   // "replace", replaceSymbol,
   // "search", searchSymbol,
   // "split, splitSymbol,
   "toPrimitive", toPrimitiveSymbol,
   // TODO(dslomov, caitp): Currently defined in harmony-tostring.js ---
   // Move here when shipping
   // "toStringTag", toStringTagSymbol,
@@ skipping 18 matching lines @@
 ]);
 
 // -------------------------------------------------------------------
 // Exports
 
 utils.Export(function(to) {
   to.SymbolToString = SymbolToString;
 })
 
 })