Add List.insert.

runtime/lib/array.dart

Index: runtime/lib/array.dart
diff --git a/runtime/lib/array.dart b/runtime/lib/array.dart
index eb7bc70760ff48b847726524943831e8409efbb8..dca6c1505fae6f502a746830dbdc407d7359b403 100644
--- a/runtime/lib/array.dart
+++ b/runtime/lib/array.dart
@@ -24,6 +24,11 @@ class _ObjectArray<E> implements List<E> {
                             int count)
       native "ObjectArray_copyFromObjectArray";
 
+  void insert(int index, E element) {
+    throw new UnsupportedError(
+        "Cannot add to a non-extendable array");
+  }
+
   E removeAt(int index) {
     throw new UnsupportedError(
         "Cannot remove element of a non-extendable array");
@@ -265,6 +270,11 @@ class _ImmutableArray<E> implements List<E> {
 
   int get length native "ObjectArray_getLength";
 
+  void insert(int index, E element) {
+    throw new UnsupportedError(
+        "Cannot add to an immutable array");
+  }
+
   E removeAt(int index) {
     throw new UnsupportedError(
         "Cannot modify an immutable array");