Presubmit test for overriding properties generated by Polymer mixin shim

chrome/browser/web_dev_style/css_checker_test.py

 #!/usr/bin/env python
 # Copyright 2015 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.
 
 import css_checker
 from os import path as os_path
 import re
 from sys import path as sys_path
 import unittest
@@ skipping 259 matching lines @@
     transform: two .1s; (replace with 100ms)""")
 
   def testCssNoDataUrisInSourceFiles(self):
     self.VerifyContentsProducesOutput("""
 img {
   background: url( data:image/jpeg,4\/\/350|\/|3|2 );
 }""", """
 - Don't use data URIs in source files. Use grit instead.
     background: url( data:image/jpeg,4\/\/350|\/|3|2 );""")
 
+  def testCssNoMixinShims(self):
+    self.VerifyContentsProducesOutput("""
+:host {
+  --good-property: red;
+  --not-okay-mixin_-_not-okay-property: green;
+}""", """
+- Don't override custom properties created by Polymer's mixin shim. Set \
+mixins or documented custom properties directly.
+    --not-okay-mixin_-_not-okay-property: green;""")
+
   def testCssNoQuotesInUrl(self):
     self.VerifyContentsProducesOutput("""
 img {
   background: url('chrome://resources/images/blah.jpg');
   background: url("../../folder/hello.png");
 }""", """
 - Use single quotes (') instead of double quotes (") in strings.
     background: url("../../folder/hello.png");
 
 - Don't use quotes in url().
@@ skipping 178 matching lines @@
   </style>
 </head>
 </html>""", """
 - Colons (:) should have a space after them.
     flex-direction:column;
 """, filename='test.html')
 
 
 if __name__ == '__main__':
   unittest.main()