Fixes for a number of dart analyzer issues in dart:html.

sdk/lib/html/html_common/filtered_element_list.dart

Index: sdk/lib/html/html_common/filtered_element_list.dart
diff --git a/sdk/lib/html/html_common/filtered_element_list.dart b/sdk/lib/html/html_common/filtered_element_list.dart
index 39a311c3b873de51e138a8c0c2e215aa204d1dcd..776d2fd31f1578724302da971a166254cbd779f0 100644
--- a/sdk/lib/html/html_common/filtered_element_list.dart
+++ b/sdk/lib/html/html_common/filtered_element_list.dart
@@ -114,6 +114,8 @@ class FilteredElementList implements List {
   }
   bool every(bool f(Element element)) => _filtered.every(f);
   bool any(bool f(Element element)) => _filtered.any(f);
+  List<Element> toList() => new List<Element>.from(this);
+  Set<Element> toSet() => new Set<Element>.from(this);
   Element firstMatching(bool test(Element value), {Element orElse()}) {
     return _filtered.firstMatching(test, orElse: orElse);
   }
@@ -126,7 +128,7 @@ class FilteredElementList implements List {
     return _filtered.singleMatching(test);
   }
 
-  E elementAt(int index) {
+  Element elementAt(int index) {
     return this[index];
   }