Morph the APIs for Node, ParentNode, and Element closer to the specs

sky/tests/editing/delete_block_contents.sky

 <sky>
 This test verifies that the height of an editable block remains the same after adding block elements and removing them.
 <div contenteditable="true" style="border: solid blue" id="test"></div>
 <script>
 import "../resources/third_party/unittest/unittest.dart";
 import "../resources/unit.dart";
 
 import "dart:async";
 import "dart:sky";
 
 void main() {
   initUnit();
 
   test("remains the same after adding block elements and removing them", () {
     var elem = document.getElementById("test");
     var originalHeight = elem.offsetHeight;
     var d = elem.appendChild(document.createElement('div'));
     d.appendChild(new Text('aaa'));
     d = elem.appendChild(document.createElement('div'));
     d.appendChild(new Text('bbb'));
     var newHeight = elem.offsetHeight;
 
-    while (elem.firstChild != null) {
-       elem.removeChild(elem.firstChild);
-    }
+    elem.removeChildren();
 
     new Timer(Duration.ZERO, expectAsync(() {
       expect(elem.offsetHeight, equals(originalHeight));
     }));
   });
 }
 </script>
 </sky>