Move more builtin files to strict mode.

src/date.js

 // Copyright 2006-2008 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.
 
+"use strict";
+
 // This file relies on the fact that the following declarations have been made
 // in v8natives.js:
 // var $isFinite = GlobalIsFinite;
 
 var $Date = global.Date;
 
 // -------------------------------------------------------------------
 
 // This file contains date support implemented in JavaScript.
 
@@ skipping 794 matching lines @@
     "toGMTString", DateToGMTString,
     "toUTCString", DateToUTCString,
     "getYear", DateGetYear,
     "setYear", DateSetYear,
     "toISOString", DateToISOString,
     "toJSON", DateToJSON
   ));
 }
 
 SetUpDate();