Use _downcast and _cast instead of "as" to reduce dart2js output size.

tools/dom/src/WrappedList.dart

Index: tools/dom/src/WrappedList.dart
diff --git a/tools/dom/src/WrappedList.dart b/tools/dom/src/WrappedList.dart
index 4cd72260bd33423b45a0b643738576f187999898..c06e8dc923d8016355d1883c725f7ee6764dd1c6 100644
--- a/tools/dom/src/WrappedList.dart
+++ b/tools/dom/src/WrappedList.dart
@@ -30,13 +30,13 @@ class _WrappedList<E extends Node> extends ListBase<E>
 
   // List APIs
 
-  E operator [](int index) => _list[index] as E;
+  E operator [](int index) => _downcast/*<Node, E>*/(_list[index]);
 
   void operator []=(int index, E value) { _list[index] = value; }
 
   set length(int newLength) { _list.length = newLength; }
 
-  void sort([int compare(E a, E b)]) { _list.sort((Node a, Node b) => compare(a as E, b as E)); }
+  void sort([int compare(E a, E b)]) { _list.sort((Node a, Node b) => compare(_downcast/*<Node, E>*/(a), _downcast/*<Node, E>*/(b))); }
 
   int indexOf(Object element, [int start = 0]) => _list.indexOf(element, start);
 
@@ -44,7 +44,7 @@ class _WrappedList<E extends Node> extends ListBase<E>
 
   void insert(int index, E element) => _list.insert(index, element);
 
-  E removeAt(int index) => _list.removeAt(index) as E;
+  E removeAt(int index) => _downcast/*<Node, E>*/(_list.removeAt(index));
 
   void setRange(int start, int end, Iterable<E> iterable, [int skipCount = 0]) {
     _list.setRange(start, end, iterable, skipCount);
@@ -66,7 +66,7 @@ class _WrappedList<E extends Node> extends ListBase<E>
 /**
  * Iterator wrapper for _WrappedList.
  */
-class _WrappedIterator<E> implements Iterator<E> {
+class _WrappedIterator<E extends Node> implements Iterator<E> {
   Iterator<Node> _iterator;
 
   _WrappedIterator(this._iterator);
@@ -75,5 +75,8 @@ class _WrappedIterator<E> implements Iterator<E> {
     return _iterator.moveNext();
   }
 
-  E get current => _iterator.current as E;
+  E get current => _downcast/*<Node, E>*/(_iterator.current);
 }
+
+// ignore: STRONG_MODE_DOWN_CAST_COMPOSITE
+/*=To*/ _downcast/*<From, To extends From>*/(dynamic /*=From*/ x) => x;