Change Set.add to return a boolean indicating success.

corelib/src/implementation/hash_map_set.dart

Index: corelib/src/implementation/hash_map_set.dart
diff --git a/corelib/src/implementation/hash_map_set.dart b/corelib/src/implementation/hash_map_set.dart
index c538951fff31beb62674103b8c7517339c0c8ded..8dc54a9f64e8bafe588c1f62d74f9dda8e67ed51 100644
--- a/corelib/src/implementation/hash_map_set.dart
+++ b/corelib/src/implementation/hash_map_set.dart
@@ -292,8 +292,10 @@ class HashSetImplementation<E extends Hashable> implements HashSet<E> {
     _backingMap.clear();
   }
 
-  void add(E value) {
+  bool add(E value) {
+    if (_backingMap.containsKey(value)) return false;
     _backingMap[value] = value;
+    return true;
   }
 
   bool contains(E value) {