Handle toString on DOM types

tests/lib_strong/html/private_extension_member_test.dart

Index: tests/lib_strong/html/private_extension_member_test.dart
diff --git a/tests/lib_strong/html/private_extension_member_test.dart b/tests/lib_strong/html/private_extension_member_test.dart
index 0425e95eb5e74c9ce6b6c5de83931bb9468ed36d..4512d9edf33a7d72d4cb6e8fabaee2b24f3c1b0c 100644
--- a/tests/lib_strong/html/private_extension_member_test.dart
+++ b/tests/lib_strong/html/private_extension_member_test.dart
@@ -13,6 +13,6 @@ main() {
   //
   // The createFragment() method sets `_innerHtml` on the element, so we use it
   // as a test case.
-  Expect.equals("Instance of 'DocumentFragment'",
+  Expect.equals("[object DocumentFragment]",

[vsm, 2017/06/23 13:59:47]

Note: this test was added (only to strong) for DDC.  The update matches Dartium
behavior (and I think dart2js).

       new BRElement().createFragment("Hi").toString());
 }