[es6] Move builtin constructors for primitives to strict mode.

src/string.js

 // Copyright 2012 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) {
 
 %CheckIsBootstrapping();
 
 // -------------------------------------------------------------------
 // Imports
@@ skipping 18 matching lines @@
   RegExpExecNoTests = from.RegExpExecNoTests;
   RegExpLastMatchInfo = from.RegExpLastMatchInfo;
   SymbolToString = from.SymbolToString;
   ToNumber = from.ToNumber;
   ToString = from.ToString;
 });
 
 //-------------------------------------------------------------------
 
 function StringConstructor(x) {
+  // TODO(bmeurer): Move this to toplevel.
+  "use strict";
   if (%_ArgumentsLength() == 0) x = '';
   if (%_IsConstructCall()) {
     %_SetValueOf(this, TO_STRING_INLINE(x));
   } else {
     return IS_SYMBOL(x) ?
         %_CallFunction(x, SymbolToString) : TO_STRING_INLINE(x);
   }
 }
 
 
@@ skipping 1169 matching lines @@
   to.StringLastIndexOf = StringLastIndexOfJS;
   to.StringMatch = StringMatchJS;
   to.StringReplace = StringReplace;
   to.StringSlice = StringSlice;
   to.StringSplit = StringSplitJS;
   to.StringSubstr = StringSubstr;
   to.StringSubstring = StringSubstring;
 });
 
 })