WebUI Polymer3: Handle paths to same folder in polymer_modulizer.py.

<link rel="import" src="foo.html">
needs to be converted to
import './foo.m.js';

Where the "./" prefix is added, otherwise the following error is thrown at
runtime.

Uncaught TypeError: Failed to resolve module specifier "foo.m.js". Relative
references must start with either "/", "./", or "../".

This was discovered while porting md_select_css.html to Polymer3, which
happens to trigger this case.

Bug: 965770
Change-Id: Id22bcb70a2d1bb4035b33b54dbde87bf0321982c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1681343
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Auto-Submit: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#673414}

tools/polymer/polymer.py

@@ -79,6 +79,11 @@ def _rewrite_dependency_path(dep):
 
   dep = dep.replace(r'.html', '.m.js')
 
+  # Prepend "./" if |dep| refers to the same folder as the processed file. This
+  # prefix is necessary for JS modules paths.
+  if not re.match('chrome://', dep) and os.path.dirname(dep) == '':
+    dep = './' + dep
+
   return dep
 
 

tools/polymer/tests/custom_style.html

 <link rel="import" href="../../html/polymer.html">
 
+<link rel="import" href="foo.html">
+
 <custom-style>
   <style>
     html {

tools/polymer/tests/custom_style_expected.js

 import {Polymer, html} from 'chrome://resources/polymer/v3_0/polymer/polymer_bundled.min.js';
+import './foo.m.js';
 const $_documentContainer = document.createElement('template');
 $_documentContainer.innerHTML = `
 <custom-style>

tools/polymer/tests/dom_module.html

@@ -2,6 +2,7 @@
 
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-behaviors/paper-ripple-behavior.html">
 <link rel="import" href="../shared_vars_css.html">
+<link rel="import" href="foo.html">
 
 <dom-module id="cr-test-foo">
   <template>

tools/polymer/tests/dom_module_expected.js

 import {Polymer, html} from 'chrome://resources/polymer/v3_0/polymer/polymer_bundled.min.js';
 import {PaperRippleBehavior} from 'chrome://resources/polymer/v3_0/paper-behaviors/paper-ripple-behavior.js';
 import '../shared_vars_css.m.js';
+import './foo.m.js';
 
 Polymer({
   _template: html`

tools/polymer/tests/style_module_expected.js

 import {Polymer, html} from 'chrome://resources/polymer/v3_0/polymer/polymer_bundled.min.js';
-import 'some_other_style.m.js';
+import './some_other_style.m.js';
 const styleElement = document.createElement('dom-module');
 styleElement.innerHTML = `
   <template>