Make the analyzer happy about collections.

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 8658733b67f9cc1839c0952741ce94d0f02893dd..b363659f7139840333cc1516e043e70559b5b694 100644
--- a/sdk/lib/collection/hash_set.dart
+++ b/sdk/lib/collection/hash_set.dart
@@ -25,6 +25,9 @@ abstract class _HashSetBase<E> extends Collection<E> implements Set<E> {
     return true;
   }
 
+  /** Create a new Set of the same type as this. */
+  Set _newSet();
+
   Set<E> intersection(Set<E> other) {
     Set<E> result = _newSet();
     if (other.length < this.length) {