Add "last" setter to List.

runtime/lib/array.dart

Index: runtime/lib/array.dart
diff --git a/runtime/lib/array.dart b/runtime/lib/array.dart
index dd3e03fc33fb934e6eb2a3cc9b93eb0ddf503675..5494f6b1f79515dcaa52a8130b6ad54d2ceb061a 100644
--- a/runtime/lib/array.dart
+++ b/runtime/lib/array.dart
@@ -243,6 +243,11 @@ class _List<E> implements List<E> {
     throw IterableElementError.noElement();
   }
 
+  void set last(E value) {
+    if (length == 0) throw IterableElementError.noElement();
+    this[length - 1] = value;
+  }
+
   E get single {
     if (length == 1) return this[0];
     if (length == 0) throw IterableElementError.noElement();
@@ -287,6 +292,10 @@ class _ImmutableList<E> implements List<E> {
     throw UnmodifiableListError.change();
   }
 
+  void set last(E value) {
+    throw UnmodifiableListError.change();
+  }
+
   int get length native "List_getLength";
 
   void insert(int index, E element) {