Handle case where there are no elements in the template

Always put `<link rel="import" type="css">` styles at the top

lib/mixins/element-mixin.html

@@ -247,17 +247,22 @@
      * @private
      */
     function processElementStyles(klass, template, is, baseURI) {
-      const styles = Polymer.StyleGather.stylesFromModuleImports(is).concat(
-        Polymer.StyleGather.stylesFromTemplate(template));
       const templateStyles = template.content.querySelectorAll('style');
+      const stylesWithImports = Polymer.StyleGather.stylesFromTemplate(template);
+      // insert styles from <link rel="import" type="css"> at the top of the template
+      const linkedStyles = Polymer.StyleGather.stylesFromModuleImports(is);
+      const firstTemplateChild = template.content.firstElementChild;
+      for (let idx = 0; idx < linkedStyles.length; idx++) {
+        let s = linkedStyles[idx];
+        s.textContent = klass._processStyleText(s.textContent, baseURI);
+        template.content.insertBefore(s, firstTemplateChild);
+      }
       // keep track of the last "concrete" style in the template we have encountered
       let templateStyleIndex = 0;
       // ensure all gathered styles are actually in this template.
-      for (let i=0; i < styles.length; i++) {
-        let s = styles[i];
-        // if no styles in the template, fall back to first element child of template
-        let templateStyle = templateStyles.length ?
-          templateStyles[templateStyleIndex] : template.content.firstElementChild;
+      for (let i = 0; i < stylesWithImports.length; i++) {
+        let s = stylesWithImports[i];
+        let templateStyle = templateStyles[templateStyleIndex];
         // 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-import-host.css

@@ -0,0 +1,13 @@
+/*
+@license
+Copyright (c) 2017 The Polymer Project Authors. All rights reserved.
+This code may only be used under the BSD style license found at http://polymer.github.io/LICENSE.txt
+The complete set of authors may be found at http://polymer.github.io/AUTHORS.txt
+The complete set of contributors may be found at http://polymer.github.io/CONTRIBUTORS.txt
+Code distributed by Google as part of the polymer project is also
+subject to an additional IP rights grant found at http://polymer.github.io/PATENTS.txt
+*/
+:host {
+  display: block;
+  border: 1px solid black;
+}

test/unit/styling-import-host2.css

@@ -0,0 +1,13 @@
+/*
+@license
+Copyright (c) 2017 The Polymer Project Authors. All rights reserved.
+This code may only be used under the BSD style license found at http://polymer.github.io/LICENSE.txt
+The complete set of authors may be found at http://polymer.github.io/AUTHORS.txt
+The complete set of contributors may be found at http://polymer.github.io/CONTRIBUTORS.txt
+Code distributed by Google as part of the polymer project is also
+subject to an additional IP rights grant found at http://polymer.github.io/PATENTS.txt
+*/
+:host {
+  display: block;
+  border: 2px solid black;
+}

test/unit/styling-scoped.html

@@ -716,10 +716,9 @@
   </dom-module>
 
   <dom-module id="x-only-link-style">
-    <link rel="import" type="css" href="styling-import1.css">
-    <template>
-      <div id="one"></div>
-    </template>
+    <link rel="import" type="css" href="styling-import-host.css">
+    <link rel="import" type="css" href="styling-import-host2.css">
+    <template></template>
     <script>
       addEventListener('WebComponentsReady', () => {
         class XOnlyLink extends Polymer.Element {
@@ -1003,7 +1002,7 @@
     test('elements without styles work', function() {
       var e = document.createElement('x-only-link-style');
       document.body.appendChild(e);
-      assertComputed(e.$.one, '1px');
+      assertComputed(e, '2px');
     });
 
   });