Another try at placating the type checking for FilteredElementList

BUG=

Review URL: https://codereview.chromium.org//1230313008 .

Author: alan-knight

sdk/lib/html/html_common/filtered_element_list.dart

@@ -28,9 +28,10 @@ class FilteredElementList extends ListBase<Element> implements NodeListWrapper {
   // with externally to this class.
   //
   // We can't use where directly because the types don't agree and there's
-  // no way to cast it, so take advantage of being in the SDK.
+  // no way to cast it, so take advantage of being in the SDK to construct
+  // a WhereIterable directly. Even so it has to be of dynamic.
   Iterable<Element> get _iterable =>
-      new WhereIterable<Element>(_childNodes, (n) => n is Element);
+      new WhereIterable(_childNodes, (n) => n is Element);
   List<Element> get _filtered =>
       new List<Element>.from(_iterable, growable: false);
 
@@ -136,7 +137,7 @@ class FilteredElementList extends ListBase<Element> implements NodeListWrapper {
   bool remove(Object element) {
     if (element is! Element) return false;
     if (contains(element)) {
-      element.remove();
+      (element as Element).remove(); // Placate the type checker
       return true;
     } else {
       return false;