WebUI Polymer 3: Update polymer script for arrow wrapper functions

Update tools/polymer/polymer.py so that files with content wrapped in:
(() => {
...
})()
are processed the same way as files with content wrapped in:
(function() {
...
})()

Also add a test.

Bug: 965770
Change-Id: I55e7e2bf76e243fbac100ac8e1368fe599df03df
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1775747Reviewed-by: Dan Beam <dbeam@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#691800}

tools/polymer/polymer.py

@@ -300,6 +300,7 @@ def _process_dom_module(js_file, html_file):
   js_imports = _generate_js_imports(html_file)
 
   IIFE_OPENING = '(function() {\n'
+  IIFE_OPENING_ARROW = '(() => {\n'
   IIFE_CLOSING = '})();'
 
   with open(js_file) as f:
@@ -310,7 +311,7 @@ def _process_dom_module(js_file, html_file):
 
   for i, line in enumerate(lines):
     if not imports_added:
-      if line.startswith(IIFE_OPENING):
+      if line.startswith(IIFE_OPENING) or line.startswith(IIFE_OPENING_ARROW):
         # Replace the IIFE opening line with the JS imports.
         line = '\n'.join(js_imports) + '\n\n'
         imports_added = True

tools/polymer/polymer_test.py

@@ -59,6 +59,13 @@ class PolymerModulizerTest(unittest.TestCase):
         'dom-module', 'dom_module.html', 'dom_module_iife.js',
         'dom_module_iife.m.js', 'dom_module_iife_expected.js')
 
+  # Test case where HTML is extracted from a Polymer2 <dom-module> that is
+  # wrapped in an arrow IIFE function.
+  def testDomModuleIifeArrow(self):
+    self._run_test(
+        'dom-module', 'dom_module.html', 'dom_module_iife_arrow.js',
+        'dom_module_iife_arrow.m.js', 'dom_module_iife_expected.js')
+
   # Test case where HTML is extracted from a Polymer2 style module.
   def testStyleModule(self):
     self._run_test(

tools/polymer/tests/dom_module_iife_arrow.js

+(() => {
+const foo = 'foo';
+
+Polymer({
+  is: 'cr-test-foo',
+  behaviors: [Polymer.PaperRippleBehavior],
+});
+})();