Implement RemoveAt and RemoveRange on ListProxy

Fixes problems when running on Dart 1.24
diff --git a/lib/src/list_proxy.dart b/lib/src/list_proxy.dart
index b2fe512..63aeea6 100644
--- a/lib/src/list_proxy.dart
+++ b/lib/src/list_proxy.dart
@@ -38,4 +38,10 @@
   void insertAll(int index, Iterable<E> iterable) {
     _list.insertAll(index, iterable);
   }
+
+  E removeAt(int index) => _list.removeAt(index);
+
+  void removeRange(int start, int length) {
+    _list.removeRange(start, length);
+  }
 }