browser.js
function loadSync() {
    debug('Loading environment scripts:');
    var a11ySuiteScriptPath = 'web-component-tester/data/a11ySuite.js';
    var scripts = get('environmentScripts');
    var a11ySuiteWillBeLoaded = window.__generatedByWct || scripts.indexOf(a11ySuiteScriptPath) > -1;
    // We can't inject a11ySuite when running the npm version because it is a
    // module-based script that needs `<script type=module>` and compilation
    // for browsers without module support.
    if (!a11ySuiteWillBeLoaded && !window.__wctUseNpm) {
        // wct is running as a bower dependency, load a11ySuite from data/
        scripts.push(a11ySuiteScriptPath);
    }
    scripts.forEach(function (path) {
        var url = expandUrl(path, get('root'));
        debug('Loading environment script:', url);
        // Synchronous load.
        document.write('<script src="' + encodeURI(url) +
            '"></script>'); // jshint ignore:line
    });
    debug('Environment scripts loaded');
    var imports = get('environmentImports');
    imports.forEach(function (path) {
        var url = expandUrl(path, get('root'));
        debug('Loading environment import:', url);
        // Synchronous load.
        document.write('<link rel="import" href="' + encodeURI(url) +
            '">'); // jshint ignore:line
    });
    debug('Environment imports loaded');
}
function loadSync() {
    debug('Loading environment scripts:');
    var a11ySuiteScriptPath = 'web-component-tester/data/a11ySuite.js';
    var scripts = get('environmentScripts');
    var a11ySuiteWillBeLoaded = window.__generatedByWct || scripts.indexOf(a11ySuiteScriptPath) > -1;
    // We can't inject a11ySuite when running the npm version because it is a
    // module-based script that needs `<script type=module>` and compilation
    // for browsers without module support.
    if (!a11ySuiteWillBeLoaded && !window.__wctUseNpm) {
        // wct is running as a bower dependency, load a11ySuite from data/
        scripts.push(a11ySuiteScriptPath);
    }
    scripts.forEach(function (path) {
        var url = expandUrl(path, get('root'));
        debug('Loading environment script:', url);
        // Synchronous load.
        document.write('<script src="' + encodeURI(url) +
            '"></script>'); // jshint ignore:line
    });
    debug('Environment scripts loaded');
    var imports = get('environmentImports');
    imports.forEach(function (path) {
        var url = expandUrl(path, get('root'));
        debug('Loading environment import:', url);
        // Synchronous load.
        document.write('<link rel="import" href="' + encodeURI(url) +
            '">'); // jshint ignore:line
    });
    debug('Environment imports loaded');
}
Loads all environment scripts ...synchronously ...after us.