Add SplayTreeSet.

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 74a25d502d94e81e1fa80d88176e8f43d28ed4b2..10d74ba5472e0fc8aa6d1640fb513018ef21b9c7 100644
--- a/sdk/lib/collection/hash_set.dart
+++ b/sdk/lib/collection/hash_set.dart
@@ -20,14 +20,8 @@ abstract class _HashSetBase<E> extends IterableBase<E> implements Set<E> {
 
   Set<E> intersection(Set<Object> other) {
     Set<E> result = _newSet();
-    if (other.length < this.length) {
-      for (var element in other) {
-        if (this.contains(element)) result.add(element);
-      }
-    } else {
-      for (E element in this) {
-        if (other.contains(element)) result.add(element);
-      }
+    for (var element in this) {
+      if (other.contains(element)) result.add(element);
     }
     return result;
   }