remove type annotation from AttributeMapWrapper

for some reason dart2js doesn't like this
BUG=

Review URL: https://codereview.chromium.org//33173004

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@29100 260f80e4-7a28-3924-810f-c04153c831b5

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');