Set.add returns true if item has been added, otherwise false

sdk/lib/core/set.dart

Index: sdk/lib/core/set.dart
diff --git a/sdk/lib/core/set.dart b/sdk/lib/core/set.dart
index f8af64c95f67ab3349dad15bdadf20668669bfe0..c22d0d73e677d9ef353d5278375805d3ab6b9f7d 100644
--- a/sdk/lib/core/set.dart
+++ b/sdk/lib/core/set.dart
@@ -59,11 +59,11 @@ abstract class Set<E> extends IterableBase<E> implements EfficientLength {
   bool contains(Object value);
 
   /**
-   * Adds [value] into the set.
+   * Adds [value] into the set. Returns `true` if [value] was added to the set.
    *
-   * The method has no effect if [value] is already in the set.
+   * If [value] already exists, the set is not changed and `false` is returned.
    */
-  void add(E value);
+  bool add(E value);
 
   /**
    * Adds all of [elements] to this Set.