From Chrome v32, it supports Promise. It's easy to chrome.tabs.executeScript multiple scripts.

Usage example:

new ScriptExecution(tab.id)
    .executeScripts("js/jquery.js", "js/script.js")
    .then(s => s.executeCodes('console.log("executes code...")'))
    .then(s => s.injectCss("css/style.css"))
    .then(s => console.log('done'));

OR

let executer = new ScriptExecution(tab.id);

executer.executeScripts("js/jquery.js", "js/script.js")
    .then(s => s.executeCodes('console.log("executes code...")'))
    .then(s => s.injectCss("css/style.css"))
    .then(s => console.log('done'));

More info: http://stackoverflow.com/questions/21535233/injecting-multiple-scripts-through-executescript-in-google-chrome/34456163#34456163