Make built-ins strict mode conforming, and support a --use-strict flag.

src/date.js

Index: src/date.js
diff --git a/src/date.js b/src/date.js
index 999009e8639089bef4ef59493ebda191d2d29ebb..8c51a931e4bfda2140fb281d5cb15937adbf6482 100644
--- a/src/date.js
+++ b/src/date.js
@@ -28,17 +28,16 @@
 
 // This file relies on the fact that the following declarations have been made
 // in v8natives.js:
-// const $isFinite = GlobalIsFinite;
+// var $isFinite = GlobalIsFinite;
 
 // -------------------------------------------------------------------
 
 // This file contains date support implemented in JavaScript.
 
-
 // Keep reference to original values of some global properties.  This
 // has the added benefit that the code in this file is isolated from
 // changes to these properties.
-const $Date = global.Date;
+var $Date = global.Date;
 
 // Helper function to throw error.
 function ThrowDateTypeError() {