Change Set.add to return a boolean indicating success.

corelib/src/set.dart

Index: corelib/src/set.dart
diff --git a/corelib/src/set.dart b/corelib/src/set.dart
index 5893cf22ff2307e37448770cded7a2ead97a6905..6f1d396d1a02abe2d13acd83ddcf2386ae946613 100644
--- a/corelib/src/set.dart
+++ b/corelib/src/set.dart
@@ -21,10 +21,10 @@ interface Set<E> extends Collection<E>
   bool contains(E value);
 
   /**
-   * Adds [value] into the set. The method has no effect if
-   * [value] was already in the set.
+   * Adds [value] into the set. Returns true if [value] was not in the set.
+   * The method has no effect if [value] was already in the set.
    */
-  void add(E value);
+  bool add(E value);
 
   /**
    * Removes [value] from the set. Returns true if [value] was