Google's Angular docs turned me onto the HTML trick of linking to external files without specifying the scheme/protocol (e.g., http or https).
Don't do this:
The browser will implicitly load the external resource using the same scheme as the page.
This avoids dynamically loading scripts or rewriting HTML to support hosting on both HTTP and HTTPS.
HTTPS is becoming much more ubiquitous thanks to better TLS scalability and privacy concerns of HTTP over open networks like WiFi hot spots. Even if your site isn't using HTTPS today, this simple trick today makes it easier to "flip the swtich" tomorrow with HTTPS support.
Keep in mind a few CDN operators (e.g., http://code.highcharts.com/) will overlook this trick even though they support both HTTP and HTTPS.