Moving ListBase, FixedLengthListMixin and UmodifiableListMixin to collection.

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

Index: tools/dom/templates/html/impl/impl_Element.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_Element.darttemplate b/tools/dom/templates/html/impl/impl_Element.darttemplate
index 92f357da4f6197ed6ea8b67579cec35c19432f87..d76b46b0036bb2aef78adb5b5b284be9aecf2ef9 100644
--- a/tools/dom/templates/html/impl/impl_Element.darttemplate
+++ b/tools/dom/templates/html/impl/impl_Element.darttemplate
@@ -6,7 +6,7 @@ part of $LIBRARYNAME;
 
 // TODO(jacobr): use _Lists.dart to remove some of the duplicated
 // functionality.
-class _ChildrenElementList implements List {
+class _ChildrenElementList implements List<Element> {
   // Raw Element.
   final Element _element;
   final HtmlCollection _childElements;
@@ -64,19 +64,19 @@ class _ChildrenElementList implements List {
   }
 
   String join([String separator]) {
-    return IterableMixinWorkaround.joinList(this, separator);
+    return _childElements.join(separator);
   }
 
   Iterable map(f(Element element)) {
-    return IterableMixinWorkaround.mapList(this, f);
+    return _childElements.map(f);
   }
 
   Iterable<Element> where(bool f(Element element)) {
-    return IterableMixinWorkaround.where(this, f);
+    return _childElements.where(f);
   }
 
   Iterable expand(Iterable f(Element element)) {
-    return IterableMixinWorkaround.expand(this, f);
+    return _childElements.expand(f);
   }
 
   bool get isEmpty {