Adding tests for Element & Node lists insert method.

tests/html/element_test.dart

Index: tests/html/element_test.dart
diff --git a/tests/html/element_test.dart b/tests/html/element_test.dart
index 35b5b2ec24d33796b017ae6fd2264457fcd394e6..644afcdec185cf57e75a2f63e5dd22e9e4ad392c 100644
--- a/tests/html/element_test.dart
+++ b/tests/html/element_test.dart
@@ -465,6 +465,18 @@ main() {
       expect(el.children[5], isHeadingElement);
     });
 
+    test('insert', () {
+      var element = new DivElement();
+      element.children.insert(0, new BRElement());
+      expect(element.children[0], isBRElement);
+      element.children.insert(0, new HRElement());
+      expect(element.children[0], isHRElement);
+      element.children.insert(1, new ImageElement());
+      expect(element.children[1], isImageElement);
+      element.children.insert(element.children.length, new InputElement());
+      expect(element.children.last, isInputElement);
+    });
+
     test('clear', () {
       var el = makeElementWithChildren();
       el.children.clear();