babel: Enable loose mode.

Due to try-catch deoptimization, Babel strict mode for…of loops run
about 5× slower in Firefox than Babel loose mode for…of, native
for…of, or forEach (which are all about the same speed).  Chrome
doesn’t seem to care.

For some reason we need to explicitly add the core-js Symbol polyfill
near the beginning of the common bundle.  Otherwise it gets loaded at
the wrong time and the Casper tests fail.

Signed-off-by: Anders Kaseorg <anders@zulipchat.com>

babel.config.js

@@ -4,6 +4,7 @@ module.exports = {
             "@babel/preset-env",
             {
                 corejs: 3,
+                loose: true,  // Loose mode for…of loops are 5× faster in Firefox
                 useBuiltIns: "usage",
             },
         ],

static/js/bundles/common.js

@@ -1,4 +1,5 @@
 import "core-js/features/promise";
+import "core-js/features/symbol";
 import "jquery/dist/jquery.js";
 import "underscore/underscore.js";
 import "../page_params.js";

static/js/unread.js

@@ -297,7 +297,7 @@ exports.unread_topic_counter = (function () {
 
             return {
                 pretty_name: topic_name,
-                message_id: Math.max(...msgs),
+                message_id: Math.max(...Array.from(msgs)),
             };
         });