build/lib/optimize.ts/toConcatStream
function toConcatStream(src: string, bundledFileHeader: string, sources: bundle.IFile[], dest: string, fileContentMapper: (contents: string, path: string) => string): NodeJS.ReadWriteStream {
	const useSourcemaps = /\.js$/.test(dest) && !/\.nls\.js$/.test(dest);

	// If a bundle ends up including in any of the sources our copyright, then
	// insert a fake source at the beginning of each bundle with our copyright
	let containsOurCopyright = false;
	for (let i = 0, len = sources.length; i < len; i++) {
		const fileContents = sources[i].contents;
		if (IS_OUR_COPYRIGHT_REGEXP.test(fileContents)) {
			containsOurCopyright = true;
			break;
		}
	}

	if (containsOurCopyright) {
		sources.unshift({
			path: null,
			contents: bundledFileHeader
		});
	}

	const treatedSources = sources.map(function (source) {
		const root = source.path ? REPO_ROOT_PATH.replace(/\\/g, '/') : '';
		const base = source.path ? root + `/${src}` : '.';
		const path = source.path ? root + '/' + source.path.replace(/\\/g, '/') : 'fake';
		const contents = source.path ? fileContentMapper(source.contents, path) : source.contents;

		return new VinylFile({
			path: path,
			base: base,
			contents: Buffer.from(contents)
		});
	});

	return es.readArray(treatedSources)
		.pipe(useSourcemaps ? util.loadSourcemaps() : es.through())
		.pipe(concat(dest))
		.pipe(createStatsStream(dest));
}