globalCSS.js doesn't work correctly with hot reloading

[dainyl]

Steps to recreate:

run npm start

change body style in globalCSS.js to:

    body {
        font-family: Lato, Arial, "Lucida Grande", sans-serif;
        margin: 0px;
        background: black;
    }

Save file -> background is changed to black as expected

Revert body style in globalCSS.js back to:

    body {
        font-family: Lato, Arial, "Lucida Grande", sans-serif;
        margin: 0px;
    }

Save file -> background is still black whereas it should be white

[dainyl]

This bug was introduced in 290cbcc