diff --git a/gulpfile.js b/gulpfile.js
index b2ae0a1bc9..340f044e58 100644
--- a/gulpfile.js
+++ b/gulpfile.js
@@ -17,6 +17,7 @@ var rundartpackage = require('./tools/build/rundartpackage');
 var multicopy = require('./tools/build/multicopy');
 var karma = require('karma').server;
 var minimist = require('minimist');
+var es5build = require('./tools/build/es5build');
 
 var DART_SDK = require('./tools/build/dartdetect')(gulp);
 // -----------------------
@@ -67,8 +68,14 @@ var CONFIG = {
   dest: {
     js: {
       all: 'dist/js',
-      dev: 'dist/js/dev',
-      prod: 'dist/js/prod',
+      dev: {
+        es6: 'dist/js/dev/es6',
+        es5: 'dist/js/dev/es5'
+      },
+      prod: {
+        es6: 'dist/js/prod/es6',
+        es5: 'dist/js/prod/es5'
+      },
       dart2js: 'dist/js/dart2js'
     },
     dart: 'dist/dart',
@@ -131,10 +138,12 @@ var CONFIG = {
       js: {
         dev: merge(true, _COMPILER_CONFIG_JS_DEFAULT, {
           typeAssertionModule: 'rtts_assert/rtts_assert',
-          typeAssertions: true
+          typeAssertions: true,
+          outputLanguage: 'es6'
         }),
         prod: merge(true, _COMPILER_CONFIG_JS_DEFAULT, {
-          typeAssertions: false
+          typeAssertions: false,
+          outputLanguage: 'es6'
         })
       },
       dart: {
@@ -212,12 +221,12 @@ gulp.task('build/clean.docs', clean(gulp, gulpPlugins, {
 
 gulp.task('build/deps.js.dev', deps(gulp, gulpPlugins, {
   src: CONFIG.deps.js,
-  dest: CONFIG.dest.js.dev
+  dest: CONFIG.dest.js.dev.es5
 }));
 
 gulp.task('build/deps.js.prod', deps(gulp, gulpPlugins, {
   src: CONFIG.deps.js,
-  dest: CONFIG.dest.js.prod
+  dest: CONFIG.dest.js.prod.es5
 }));
 
 gulp.task('build/deps.js.dart2js', deps(gulp, gulpPlugins, {
@@ -228,24 +237,50 @@ gulp.task('build/deps.js.dart2js', deps(gulp, gulpPlugins, {
 // ------------
 // transpile
 
-gulp.task('build/transpile.js.dev', transpile(gulp, gulpPlugins, {
+gulp.task('build/transpile.js.dev.es6', transpile(gulp, gulpPlugins, {
   src: CONFIG.transpile.src.js,
   copy: CONFIG.transpile.copy.js,
-  dest: CONFIG.dest.js.dev,
-  outputExt: 'js',
+  dest: CONFIG.dest.js.dev.es6,
+  outputExt: 'es6',
   options: CONFIG.transpile.options.js.dev,
   srcFolderInsertion: CONFIG.srcFolderInsertion.js
 }));
 
-gulp.task('build/transpile.js.prod', transpile(gulp, gulpPlugins, {
+gulp.task('build/transpile.js.dev.es5', es5build({
+  src: CONFIG.dest.js.dev.es6,
+  dest: CONFIG.dest.js.dev.es5,
+  modules: 'instantiate'
+}));
+
+gulp.task('build/transpile.js.dev', function() {
+  return runSequence(
+    'build/transpile.js.dev.es6',
+    'build/transpile.js.dev.es5'
+  );
+});
+
+gulp.task('build/transpile.js.prod.es6', transpile(gulp, gulpPlugins, {
   src: CONFIG.transpile.src.js,
   copy: CONFIG.transpile.copy.js,
-  dest: CONFIG.dest.js.prod,
-  outputExt: 'js',
+  dest: CONFIG.dest.js.prod.es6,
+  outputExt: 'es6',
   options: CONFIG.transpile.options.js.prod,
   srcFolderInsertion: CONFIG.srcFolderInsertion.js
 }));
 
+gulp.task('build/transpile.js.prod.es5', es5build({
+  src: CONFIG.dest.js.prod.es6,
+  dest: CONFIG.dest.js.prod.es5,
+  modules: 'instantiate'
+}));
+
+gulp.task('build/transpile.js.prod', function() {
+  return runSequence(
+    'build/transpile.js.prod.es6',
+    'build/transpile.js.prod.es5'
+  );
+});
+
 gulp.task('build/transpile.dart', transpile(gulp, gulpPlugins, {
   src: CONFIG.transpile.src.dart,
   copy: CONFIG.transpile.copy.dart,
@@ -278,14 +313,14 @@ gulp.task('build/transpile/e2eTest.cjs', transpile(gulp, gulpPlugins, {
 
 gulp.task('build/html.js.dev', html(gulp, gulpPlugins, {
   src: CONFIG.html.src.js,
-  dest: CONFIG.dest.js.dev,
+  dest: CONFIG.dest.js.dev.es5,
   srcFolderInsertion: CONFIG.srcFolderInsertion.js,
   scriptsPerFolder: CONFIG.html.scriptsPerFolder.js
 }));
 
 gulp.task('build/html.js.prod', html(gulp, gulpPlugins, {
   src: CONFIG.html.src.js,
-  dest: CONFIG.dest.js.prod,
+  dest: CONFIG.dest.js.prod.es5,
   srcFolderInsertion: CONFIG.srcFolderInsertion.js,
   scriptsPerFolder: CONFIG.html.scriptsPerFolder.js
 }));
@@ -354,12 +389,12 @@ gulp.task('build/format.dart', rundartpackage(gulp, gulpPlugins, {
 // ------------------
 // web servers
 gulp.task('serve.js.dev', jsserve(gulp, gulpPlugins, {
-  path: CONFIG.dest.js.dev,
+  path: CONFIG.dest.js.dev.es5,
   port: 8000
 }));
 
 gulp.task('serve.js.prod', jsserve(gulp, gulpPlugins, {
-  path: CONFIG.dest.js.prod,
+  path: CONFIG.dest.js.prod.es5,
   port: 8001
 }));
 
diff --git a/package.json b/package.json
index 878841395e..dc45085667 100644
--- a/package.json
+++ b/package.json
@@ -22,6 +22,9 @@
     "node-uuid": "1.4.x"
   },
   "devDependencies": {
+    "yargs": "2.3.*",
+    "gulp-sourcemaps": "1.3.*",
+    "gulp-traceur": "0.16.*",
     "angular": "1.3.5",
     "bower": "^1.3.12",
     "canonical-path": "0.0.2",
diff --git a/tools/build/es5build.js b/tools/build/es5build.js
new file mode 100755
index 0000000000..3956867856
--- /dev/null
+++ b/tools/build/es5build.js
@@ -0,0 +1,78 @@
+#!/usr/bin/env node
+
+// Attention: This file will be distributed with our npm packages!
+var gulp = require('gulp');
+var traceur = require('gulp-traceur');
+var rename = require('gulp-rename');
+var sourcemaps = require('gulp-sourcemaps');
+var through2 = require('through2');
+var fs = require('fs');
+var path = require('path');
+
+module.exports = run;
+
+if (!module.parent) {
+  var argv = require('yargs')
+      .usage('Transpile to es5.\n\n'+
+        'Usage: $0 -m [format] -s [folder] -d [folder]')
+      .example('$0 -d tmp', 'transpile in `instantate` format to tmp/')
+      .options({
+        's': {
+          alias: 'source',
+          describe: 'source folder',
+          default: '.'
+        },
+        'd': {
+          alias: 'dest',
+          describe: 'output folder',
+          demand: true
+        },
+        'm': {
+          alias: 'modules',
+          describe: 'module format, https://github.com/google/traceur-compiler/wiki/Options-for-Compiling',
+          default: 'instantiate'
+        }
+      })
+      .help('help')
+      .wrap(40)
+      .strict()
+      .argv
+  ;
+  run({
+    src: argv.s,
+    dest: argv.d,
+    modules: argv.m
+  });
+}
+
+function run(config) {
+  var src = ['!node_modules', '!node_modules/**', './**/*.es6'];
+  gulp.src(src, {cwd: config.src})
+    .pipe(rename(function(file) {
+      file.extname = file.extname.replace('.es6', '.js');
+    }))
+    // TODO(tbosch): Using sourcemaps.init({loadMaps:true}) does not combine
+    // the sourcemaps correctly!
+    .pipe(sourcemaps.init())
+    .pipe(through2.obj(function(file, encoding, done) {
+      var self = this;
+      fs.readFile(file.path.replace('.js', '.map'), function(error, buffer) {
+        if (error) {
+          return done(error);
+        }
+        file.sourceMap = JSON.parse(buffer.toString());
+        // The filename needs to be the same as the one that is transpiled
+        // so that gulp-sourcemaps can update the mapping
+        file.sourceMap.file = file.relative;
+        file.sourceMap.sourceRoot = path.dirname(file.relative);
+        self.push(file);
+        done();
+      });
+    }))
+    .pipe(traceur({
+      modules: config.modules,
+      sourceMaps: true
+    }))
+    .pipe(sourcemaps.write('.'))
+    .pipe(gulp.dest(config.dest));
+};
diff --git a/tools/transpiler/gulp-traceur.js b/tools/transpiler/gulp-traceur.js
index 5054c5c864..e86def0a34 100644
--- a/tools/transpiler/gulp-traceur.js
+++ b/tools/transpiler/gulp-traceur.js
@@ -34,7 +34,7 @@ function gulpTraceur(options, resolveModuleName) {
 
       if (sourceMap) {
         var sourceMapFile = cloneFile(file, {
-          path: file.path.replace(/\.js$/, '.map'),
+          path: file.path.replace(/\.\w+$/, '.map'),
           contents: JSON.stringify(sourceMap)
         });
 
diff --git a/tools/transpiler/index.js b/tools/transpiler/index.js
index 7d12904bc0..022ae8be5d 100644
--- a/tools/transpiler/index.js
+++ b/tools/transpiler/index.js
@@ -4,6 +4,7 @@ var fs = require('fs');
 var glob = require('glob');
 var path = require('path');
 var traceur = require('traceur');
+var assert = require('assert');
 
 exports.RUNTIME_PATH = traceur.RUNTIME_PATH;
 var TRACEUR_PATH = traceur.RUNTIME_PATH.replace('traceur-runtime.js', 'traceur.js');
@@ -55,6 +56,11 @@ exports.compile = function compile(options, paths, source) {
     result.sourceMap = JSON.parse(sourceMapString);
   }
 
+  if (localOptions.outputLanguage === 'es6' && source.indexOf('$traceurRuntime') === -1) {
+    assert(result.js.indexOf('$traceurRuntime') === -1,
+      'Transpile to ES6 must not add references to $traceurRuntime, '
+        + inputPath + ' is transpiled to:\n' + result.js);
+  }
   return result;
 };