Issue 1772 - [JSHydra] Convert const to var for Chrome

autotest/abprewrite_source.js

 const Cc = Components.classes
 const Ci = Components.interfaces;
 const Cu = Components.utils;
 const Cr = Components.results;
 const Cl = Components.unknown;
 
 Cu.import("foo/bar.jsm");
 
 let {Timeline} = require("timeline");
 let {Utils} = require("utils");
 let {Filter, BlockingFilter} = require("filterClasses");
 
 let foo;
 let bar = 2;
 var bas;
+const FOO = "FOO";
 
 let [a, b] = foo();
 [a, b] = [1, 2];
 
 let {x: y} = foo();
 let {k1: v1, k2: v2} = foo();
 
 
 for (let v of fooList)
   alert(v);
@@ skipping 51 matching lines @@
 {
   for (var i = 0; i < 10; i++)
   {
     yield i;
   }
 };
 
 let x = (y) => y + 1;
 x = y => y + 1;
 x = (a, b) => this[a] + b;
-x = (a, b) => { return () => 1; }
+x = (a, b) => { return () => 1; }