WebUI: Fix migrated_imports for ui/webui/resources/html URLs.

This fix unblocks [1] to use the
chrome://resources/html/plural_string_proxy.html URL in
polymer_modulizer()'s |migrated_imports| parameter.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/2741118

Bug: 950041
Change-Id: I9bff52d5dddc7fe9a192afd0622d276f9aa932a1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2748494
Auto-Submit: dpapad <dpapad@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#861606}

tools/polymer/polymer.py

@@ -159,13 +159,14 @@ def _to_js_normalized(self):
         self.input_scheme = 'chrome'
       return POLYMER_V3_DIR + 'polymer/polymer_bundled.min.js'
 
+    extension = ('.js'
+                 if self.html_path_normalized in _migrated_imports else '.m.js')
+
     if re.match(r'ui/webui/resources/html/', self.html_path_normalized):
       return (self.html_path_normalized
           .replace(r'ui/webui/resources/html/', 'ui/webui/resources/js/')
-          .replace(r'.html', '.m.js'))
+          .replace(r'.html', extension))
 
-    extension = (
-        '.js' if self.html_path_normalized in _migrated_imports else '.m.js')
     return self.html_path_normalized.replace(r'.html', extension)
 
   def _to_js(self):

tools/polymer/polymer_test.py

@@ -117,6 +117,7 @@ def testDomModuleWithMigratedImports(self):
     self._additional_flags = [
       '--migrated_imports',
       'tools/polymer/tests/foo.html',
+      'ui/webui/resources/html/ignore_me.html',
     ]
     self._run_test('dom-module', 'dom_module.html', 'dom_module.js',
                    'dom_module.m.js',

tools/polymer/tests/dom_module_with_migrated_imports_expected.js

@@ -1,7 +1,7 @@
 import {Polymer, html} from '//resources/polymer/v3_0/polymer/polymer_bundled.min.js';
 
 import {PaperRippleBehavior} from '//resources/polymer/v3_0/paper-behaviors/paper-ripple-behavior.js';
-import '//resources/js/ignore_me.m.js';
+import '//resources/js/ignore_me.js';
 import '../shared_vars_css.m.js';
 import './foo.js';