Fix regression with imported css

Fix case where `<link rel="import" type="css">` is the only styling for
an element

Fixes #4986

lib/mixins/element-mixin.html

@@ -255,7 +255,9 @@
       // ensure all gathered styles are actually in this template.
       for (let i=0; i < styles.length; i++) {
         let s = styles[i];
-        let templateStyle = templateStyles[templateStyleIndex];
+        // if no styles in the template, fall back to first element child of template
+        let templateStyle = templateStyles.length ?
+          templateStyles[templateStyleIndex] : template.content.firstElementChild;
         // if the style is not in this template, it's been "included" and
         // we put a clone of it in the template before the style that included it
         if (templateStyle !== s) {

test/unit/styling-scoped.html

@@ -715,6 +715,21 @@
     </script>
   </dom-module>
 
+  <dom-module id="x-only-link-style">
+    <link rel="import" type="css" href="styling-import1.css">
+    <template>
+      <div id="one"></div>
+    </template>
+    <script>
+      addEventListener('WebComponentsReady', () => {
+        class XOnlyLink extends Polymer.Element {
+          static get is() { return 'x-only-link-style'; }
+        }
+        customElements.define(XOnlyLink.is, XOnlyLink);
+      });
+    </script>
+  </dom-module>
+
 <script>
 (function() {
   function assertComputed(element, value, property, pseudo) {
@@ -985,6 +1000,12 @@
       assertComputed(e, '5px', 'padding-top');
     });
 
+    test('elements without styles work', function() {
+      var e = document.createElement('x-only-link-style');
+      document.body.appendChild(e);
+      assertComputed(e.$.one, '1px');
+    });
+
   });
 
   if (window.ShadyDOM) {