Add Set.lookup method to get the instance in the set.

pkg/unmodifiable_collection/lib/unmodifiable_collection.dart

Index: pkg/unmodifiable_collection/lib/unmodifiable_collection.dart
diff --git a/pkg/unmodifiable_collection/lib/unmodifiable_collection.dart b/pkg/unmodifiable_collection/lib/unmodifiable_collection.dart
index ec8dbf01f9085463f2748ec0c4031ce8b2e3ca26..9cac0b386b2dfa527249752c4434dfb53e187c92 100644
--- a/pkg/unmodifiable_collection/lib/unmodifiable_collection.dart
+++ b/pkg/unmodifiable_collection/lib/unmodifiable_collection.dart
@@ -179,6 +179,7 @@ class UnmodifiableSetView<E> extends _IterableView<E>
 
   Set<E> difference(Set<E> other) => _source.difference(other);
 
+  E lookup(Object object) => _source.lookup(object);
 
   /**
    * Throws an [UnsupportedError];