support being evaluated with "use strict" prefixed

Fixes use case:

    var build = new Function('module', '"use strict";\n' + <string contents of /src/bean.js>);
    var bean = {exports:{}};
    build.call(window, bean);
    bean = bean.exports;

This use case is a minimal working example extracted from my custom module loader for the browser (think of yet another browserify).