fix for Set.add tweak in r28863

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 fcb2dc65ba7bb843b8f2e282a2b124868d9cbfde..9fd35e8a4c6db9338dcc3189d3b81a13ff54c31d 100644
--- a/pkg/unmodifiable_collection/lib/unmodifiable_collection.dart
+++ b/pkg/unmodifiable_collection/lib/unmodifiable_collection.dart
@@ -84,7 +84,9 @@ class NonGrowableListView<E> extends _IterableView<E>
    * Throws an [UnsupportedError];
    * operations that change the length of the list are disallowed.
    */
-  void add(E value) => _throw();
+  bool add(E value) {
+    _throw();
+  }
 
   /**
    * Throws an [UnsupportedError];
@@ -185,7 +187,9 @@ class UnmodifiableSetView<E> extends _IterableView<E>
    * Throws an [UnsupportedError];
    * operations that change the set are disallowed.
    */
-  bool add(E value) => _throw();
+  bool add(E value) {
+    _throw();
+  }
 
   /**
    * Throws an [UnsupportedError];