WebUI: Replace uglifyjs with uglify-es which understands ES6.

third_party/node/node_modules.py

 #!/usr/bin/env python
 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 from os import path as os_path
 
 
 NODE_MODULES = os_path.join(os_path.dirname(__file__), 'node_modules')
 
 
 def _path_in_node_modules(*args):
   return os_path.join(NODE_MODULES, *args)
 
 
 def PathToVulcanize():
   return _path_in_node_modules('vulcanize', 'bin', 'vulcanize')
 
 
 def PathToCrisper():
   return _path_in_node_modules('crisper', 'bin', 'crisper')
 
 
 def PathToPolymerCssBuild():
   return _path_in_node_modules('polymer-css-build', 'bin', 'polymer-css-build')
 
 
-def PathToUglifyJs():
- return _path_in_node_modules('uglifyjs', 'bin', 'uglifyjs')
+def PathToUglify():
+ return _path_in_node_modules('uglify-es', 'bin', 'uglifyjs')
 
 
 def PathToEsLint():
  return _path_in_node_modules('eslint', 'bin', 'eslint')