gulp & rollup without breaking gulp streams

gulpfile.js

const gulp = require('gulp');
const uglify = require('gulp-uglify');
const sourcemaps = require('gulp-sourcemaps');
const buble = require('rollup-plugin-buble');
const commonjs = require('rollup-plugin-commonjs');
const nodeResolve = require('rollup-plugin-node-resolve');
const rollup = require('rollup-stream');
const source = require('vinyl-source-stream');
const buffer = require('vinyl-buffer');

let rollupOptions = {
    entry: './src/main.js',
    sourceMap: true,
    plugins: [
        // Rolls up NodeJS modules
        nodeResolve({
            jsnext: true,
            main: true
        }),
        // "Convert CommonJS modules to ES6, so they can be included in a Rollup bundle"
        commonjs({
            include: 'node_modules/**',
            sourceMap: true
        }),
        // Lightweight babel alternative
        buble()
    ],
    external: [
        // Do not roll-up these imports
        'jquery'
    ],
    globals: {
        // When jquery is imported, name is jQuery in generated code
        jquery: 'jQuery'
    },
    // Instantly invoked function extraletterontheend
    format: 'iife',
    // Don't trim unused code (delete that code yourself.)
    treeshake: false
};

gulp.task('build', function() {
    return rollup(rollupOptions)
        .pipe(source('main.js', './src'))
        .pipe(buffer())
        .pipe(sourcemaps.init({ loadMaps: true }))
        .pipe(uglify())
        .pipe(sourcemaps.write('.'))
        .pipe(gulp.dest('./dist'));
});