var path = require('path');

module.exports = {
    entry: path.resolve(__dirname, 'lib', 'client.js'),
    output: {
        filename: 'app.js',
        path: path.resolve(__dirname, 'lib'),
    },
};

var path = require('path');

module.exports = {
    entry: [
        'babel-core/polyfill',
        path.resolve(__dirname, 'lib', 'client.js'),
    ],
    output: {
        filename: 'app.js',
        path: path.resolve(__dirname, 'lib'),
    },
};

Load babel-core/polyfill in the example (thanks to @DylanSale)