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

sky/tests/editing/delete_block_contents.sky

Index: sky/tests/editing/delete_block_contents.sky
diff --git a/sky/tests/editing/delete_block_contents.sky b/sky/tests/editing/delete_block_contents.sky
index 8552e9d1e68806e8602104d903d7bccbd58f1c56..d7c041a68d151ddf5d2942af1a0800c37ce20bc8 100644
--- a/sky/tests/editing/delete_block_contents.sky
+++ b/sky/tests/editing/delete_block_contents.sky
@@ -20,9 +20,7 @@ void main() {
     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));