Migrate dom tests to dart:html.

lib/dom/templates/html/impl/impl_Element.darttemplate

Index: lib/dom/templates/html/impl/impl_Element.darttemplate
diff --git a/lib/dom/templates/html/impl/impl_Element.darttemplate b/lib/dom/templates/html/impl/impl_Element.darttemplate
index a53ab0001f1a62d4f164e55211632d6ae1be165c..d256f5c87605c2c101a8b95cbf66f4b01ee9d029 100644
--- a/lib/dom/templates/html/impl/impl_Element.darttemplate
+++ b/lib/dom/templates/html/impl/impl_Element.darttemplate
@@ -337,10 +337,13 @@ class _ElementAttributeMap implements AttributeMap {
     if (!containsKey(key)) {
       this[key] = ifAbsent();
     }
+    return this[key];
   }
 
   String remove(String key) {
+    String value = _element.$dom_getAttribute(key);
     _element.$dom_removeAttribute(key);
+    return value;
   }
 
   void clear() {
@@ -416,9 +419,8 @@ class _DataAttributeMap implements AttributeMap {
     $dom_attributes[_attr(key)] = '$value';
   }
 
-  String putIfAbsent(String key, String ifAbsent()) {
+  String putIfAbsent(String key, String ifAbsent()) =>
     $dom_attributes.putIfAbsent(_attr(key), ifAbsent);
-  }
 
   String remove(String key) => $dom_attributes.remove(_attr(key));