Fix signature of UnmodifiableListMixin.insert to match List.insert.

sdk/lib/internal/list.dart

Index: sdk/lib/internal/list.dart
diff --git a/sdk/lib/internal/list.dart b/sdk/lib/internal/list.dart
index e642fb77e34eb0361c98edd4ee7dae006b05c450..ed5c987dd46e49a45901d5bfdccb73c70deaa7e0 100644
--- a/sdk/lib/internal/list.dart
+++ b/sdk/lib/internal/list.dart
@@ -123,7 +123,7 @@ abstract class UnmodifiableListMixin<E> implements List<E> {
   }
 
   /** This operation is not supported by an unmodifiable list. */
-  E insert(int index, E value) {
+  void insert(int index, E element) {
     throw new UnsupportedError(
         "Cannot add to an unmodifiable list");
   }