Normalize babel plugins

@inner
@param {Array} plugins the plugins to normalize
@param {Object} file the file to process
@param {BuildManager} buildManager the build manager
@return {Array}
function normalizeBabelPlugins(plugins, file, buildManager) {
    if (typeof plugins === 'function') {
        plugins = plugins(file);
    }

    plugins = plugins ? [].concat(plugins) : [];
    if (!hasBabelDepPlugin(plugins)) {
        // add npm resolve plugin
        plugins.push(DEP_PLUGIN_NAME);
    }

    return (plugins || []).map(item => {
        if (typeof item === 'string') {
            let result = BUILTIN_PLUGINS[item];
            if (typeof result === 'function') {
                return result(file, buildManager);
            }
        }
        return item;
    });
}