Change the toString method of IterableBase/IterableMixin to show some elements.

sdk/lib/collection/hash_set.dart

Index: sdk/lib/collection/hash_set.dart
diff --git a/sdk/lib/collection/hash_set.dart b/sdk/lib/collection/hash_set.dart
index 12e615e02b140a16e5c5eafbfef4295de217375d..1ed7a085965c57dd6cbebc72788074b89db4a3f5 100644
--- a/sdk/lib/collection/hash_set.dart
+++ b/sdk/lib/collection/hash_set.dart
@@ -57,7 +57,8 @@ abstract class _HashSetBase<E> extends IterableBase<E> implements Set<E> {
   }
 
   List<E> toList({bool growable: true}) {
-    List<E> result = new List<E>()..length = this.length;
+    List<E> result = growable ? (new List<E>()..length = this.length)
+                              : new List<E>(this.length);
     int i = 0;
     for (E element in this) result[i++] = element;
     return result;
@@ -65,7 +66,6 @@ abstract class _HashSetBase<E> extends IterableBase<E> implements Set<E> {
 
   Set<E> toSet() => _newSet()..addAll(this);
 
-  // TODO(zarah) Remove this, and let it be inherited by IterableBase
   String toString() => IterableMixinWorkaround.toStringIterable(this, '{', '}');
 }