Publish DomBind in Polymer. scope

Fixes #4797

Author: tomalec

lib/elements/dom-bind.html

@@ -127,6 +127,8 @@
 
     customElements.define('dom-bind', DomBind);
 
+    Polymer.DomBind = DomBind;
+
   })();
 
 </script>

test/unit/dom-bind.html

@@ -58,6 +58,13 @@
 
   <script>
 
+    suite('Polymer.DomBind class', function() {
+      test('is available', function() {
+        assert.isDefined(Polymer.DomBind);
+        assert.equal(typeof Polymer.DomBind, 'function');
+      });
+    });
+
     suite('dom-bind touched before upgrade', function() {
       test('value binds top-down', function() {
         assert.equal(earlyBoundChild.textContent, 'hi!');