remove type annotation from AttributeMapWrapper

pkg/template_binding/test/custom_element_bindings_test.dart

Index: pkg/template_binding/test/custom_element_bindings_test.dart
diff --git a/pkg/template_binding/test/custom_element_bindings_test.dart b/pkg/template_binding/test/custom_element_bindings_test.dart
index 986aabc47e625b74d59827327f105db3c925f5fd..7508a31719543c853a578c63e96406b8ea0ec9b6 100644
--- a/pkg/template_binding/test/custom_element_bindings_test.dart
+++ b/pkg/template_binding/test/custom_element_bindings_test.dart
@@ -209,7 +209,7 @@ class _MyCustomBinding extends NodeBinding {
  * and see changes that the data binding system is making to the attributes.
  */
 class WithAttrsCustomElement extends HtmlElement {
-  AttributeMapWrapper<String, String> _attributes;
+  AttributeMapWrapper _attributes;
 
   factory WithAttrsCustomElement() =>
       new Element.tag('with-attrs-custom-element');