dependencies: Use core-js for String.prototype polyfills.

It seems like the de facto standard ES polyfill library these days,
and we already depend on it through simplebar.

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

frontend_tests/zjsunit/index.js

@@ -12,7 +12,7 @@ var path = require('path');
 var fs = require('fs');
 
 global.assert = require('assert');
-require('string.prototype.codepointat/codepointat.js');
+require('core-js/features/string/code-point-at');
 
 global._ = require('underscore/underscore.js');
 var _ = global._;