Support serving ES6 JS to clients (#596)

* Support serving ES6 JS to clients

* Make es6 default dir and es5 a special dir

* Fix package building

* Fix build_frontend script

gulp/common/transform.js

@@ -1,27 +1,33 @@
 const gulpif = require('gulp-if');
-
+const uglifyjs = require('uglify-js');
+const uglifyes = require('uglify-es');
 const babel = require('gulp-babel');
-const uglify = require('gulp-uglify');
+const composer = require('gulp-uglify/composer');
 const { gulp: cssSlam } = require('css-slam');
 const htmlMinifier = require('gulp-html-minifier');
 const { HtmlSplitter } = require('polymer-build');
+const pump = require('pump');
 
-module.exports.minifyStream = function (stream) {
+module.exports.minifyStream = function (stream, es6) {
   const sourcesHtmlSplitter = new HtmlSplitter();
-  return stream
-    .pipe(sourcesHtmlSplitter.split())
-    .pipe(gulpif(/[^app]\.js$/, babel({
+  return pump([
+    stream,
+    sourcesHtmlSplitter.split(),
+    gulpif(!es6, gulpif(/[^app]\.js$/, babel({
       sourceType: 'script',
       presets: [
         ['es2015', { modules: false }]
       ]
-    })))
-    .pipe(gulpif(/\.js$/, uglify({ sourceMap: false })))
-    .pipe(gulpif(/\.css$/, cssSlam()))
-    .pipe(gulpif(/\.html$/, cssSlam()))
-    .pipe(gulpif(/\.html$/, htmlMinifier({
+    }))),
+    gulpif(/\.js$/, composer(es6 ? uglifyes : uglifyjs, console)({ sourceMap: false })),
+    gulpif(/\.css$/, cssSlam()),
+    gulpif(/\.html$/, cssSlam()),
+    gulpif(/\.html$/, htmlMinifier({
       collapseWhitespace: true,
       removeComments: true
-    })))
-    .pipe(sourcesHtmlSplitter.rejoin());
+    })),
+    sourcesHtmlSplitter.rejoin(),
+  ], (error) => {
+    if (error) console.log(error);
+  });
 };