Add test for class$ binding

Author: dfreedm

test/unit/styling-scoped.html

@@ -729,6 +729,31 @@
 </script>
   </dom-module>
 
+<script type="module">
+  import { PolymerElement, html } from '../../polymer-element.js';
+  class ClassBindingUndefined extends PolymerElement {
+    static get is() { return 'class-binding-undefined'; }
+    static get properties() {
+      return {
+        data: {
+          type: Object
+        }
+      };
+    }
+    static get template() {
+      return html`
+      <style>
+        div {
+          border: 10px solid black;
+        }
+      </style>
+      <div id="div" class$="[[data.class]]">Foo</div>
+      `;
+    }
+  }
+  customElements.define(ClassBindingUndefined.is, ClassBindingUndefined);
+</script>
+
 <script type="module">
 import { dom } from '../../lib/legacy/polymer.dom.js';
 import { flush } from '../../lib/utils/flush.js';
@@ -1015,6 +1040,13 @@
     assert.isTrue(el.classList.contains('e'));
   });
 
+  test('scoping classes are preserved when a class$ binding resolves to undefined', function() {
+    const e = document.createElement('class-binding-undefined');
+    document.body.appendChild(e);
+    const el = e.$.div;
+    assertComputed(el, '10px');
+  });
+
 });
 
 suite('double including style sheets', function() {