Make html lists print a nice toString 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 53a743c8c3c8a028f227de1e0c7312f17d4e28c6..f66460b793153fcea557be459281da56bde1e77b 100644
--- a/tests/html/element_test.dart
+++ b/tests/html/element_test.dart
@@ -616,6 +616,13 @@ main() {
   });
 
   group('functional', () {
+    test('toString', () {
+      final elems = makeElementWithChildren().children;
+      expect(elems.toString(), "[BR, IMG, INPUT]");
+      final elem = makeElement().children;
+      expect(elem.toString(), '[]');
+    });
+
     test('scrollIntoView', () {
       var child = new DivElement();
       document.body.append(child);