Add "last" setter to List.

tools/dom/src/ImmutableListMixin.dart

Index: tools/dom/src/ImmutableListMixin.dart
diff --git a/tools/dom/src/ImmutableListMixin.dart b/tools/dom/src/ImmutableListMixin.dart
index ffdcc2c98ba31f3117a89fdec6b261fd1f6f9b44..97fbdf2a8432fc40460475b00b0cac75dda4538f 100644
--- a/tools/dom/src/ImmutableListMixin.dart
+++ b/tools/dom/src/ImmutableListMixin.dart
@@ -13,7 +13,11 @@ abstract class ImmutableListMixin<E> implements List<E> {
     return new FixedSizeListIterator<E>(this);
   }
 
-  // From Collection<E>:
+  // From List<E>:
+  void set last(E value) {
+    throw new UnsupportedError("Cannot modify an immutable List.");
+  }
+
   void add(E value) {
     throw new UnsupportedError("Cannot add to immutable List.");
   }
@@ -22,7 +26,6 @@ abstract class ImmutableListMixin<E> implements List<E> {
     throw new UnsupportedError("Cannot add to immutable List.");
   }
 
-  // From List<E>:
   void sort([int compare(E a, E b)]) {
     throw new UnsupportedError("Cannot sort immutable List.");
   }