build/lib/optimize.ts/toBundleStream
function toBundleStream(src: string, bundledFileHeader: string, bundles: bundle.IConcatFile[], fileContentMapper: (contents: string, path: string) => string): NodeJS.ReadWriteStream {
	return es.merge(bundles.map(function (bundle) {
		return toConcatStream(src, bundledFileHeader, bundle.sources, bundle.dest, fileContentMapper);
	}));
}