Ensure JSON var isn't shadowed in JSC and Spidermonkey

Author: josh

lib/execjs/support/json2.js

@@ -159,12 +159,13 @@
 // Create a JSON object only if one does not already exist. We create the
 // methods in a closure to avoid creating global variables.
 
-var JSON;
-if (!JSON) {
-    JSON = {};
-}
+(function (global) {
+    if (!global.JSON) {
+        global.JSON = {};
+    }
+
+    var JSON = global.JSON;
 
-(function () {
     "use strict";
 
     function f(n) {
@@ -477,4 +478,4 @@ if (!JSON) {
             throw new SyntaxError('JSON.parse');
         };
     }
-}());
+}(this));