improve test.

test/unit/polymer-dom-content.html

@@ -1446,11 +1446,15 @@
     test('adding a document fragment clears nodes in the fragment', function() {
       var x = document.createElement('x-dist-star');
       var frag = document.createDocumentFragment();
-      frag.appendChild(document.createTextNode('hi'));
-      frag.appendChild(document.createElement('div'));
+      var t = document.createTextNode('hi');
+      var d = document.createElement('div');
+      frag.appendChild(t);
+      frag.appendChild(d);
       Polymer.dom(x).appendChild(frag);
       Polymer.dom.flush();
-      assert.equal(frag.childNodes.length, 0);
+      assert.equal(Polymer.dom(t).parentNode, x, 'logical parent wrong');
+      assert.equal(Polymer.dom(d).parentNode, x, 'logical parent wrong');
+      assert.equal(frag.childNodes.length, 0, 'fragment not empty');
     });
 
     test('adding a non-distributing node removes the node from its current location', function() {
@@ -1467,8 +1471,6 @@
 
   });
 
-
-
   suite('multi-content mutations', function() {
 
     test('remove, append first content', function() {