Correct typing of LinkedHashSet.add. Add unit test.

runtime/lib/compact_hash.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:typed_data';
 
 // Hash table with open addressing that separates the index from keys/values.
 abstract class _HashBase {
   // Each occupied entry in _index is a fixed-size integer that encodes a pair:
   //   [ hash pattern for key | index of entry in _data ]
@@ skipping 319 matching lines @@
     } else {
       current = null;
       return false;
     }
   }
 }
 
 // Set implementation, analogous to _CompactLinkedHashMap.
 class _CompactLinkedHashSet<K>
     extends SetBase<K> with _HashBase
-    implements HashSet<K>, LinkedHashSet<K> {
+    implements LinkedHashSet<K> {
 
   _CompactLinkedHashSet() {
     assert(_HashBase._UNUSED_PAIR == 0);
     _index = new Uint32List(_HashBase._INITIAL_INDEX_SIZE);
     _data = new List(_HashBase._INITIAL_INDEX_SIZE >> 1);
   }
 
   int get length => _usedData - _deletedKeys;
 
   void _rehash() {
@@ skipping 19 matching lines @@
     if (oldData != null) {
       for (int i = 0; i < oldUsed; i += 1) {
         var key = oldData[i];
         if (!_HashBase._isDeleted(oldData, key)) {
           add(key);
         }
       }
     }
   }
 
-  bool add(Object key) {
+  bool add(K key) {
     final int size = _index.length;
     final int sizeMask = size - 1;
     final int maxEntries = size >> 1;
     final int fullHash = key.hashCode;
     final int hashPattern = _HashBase._hashPattern(fullHash, _hashMask, size);
     int i = _HashBase._firstProbe(fullHash, sizeMask);
     int firstDeleted = -1;
     int pair = _index[i];
     while (pair != _HashBase._UNUSED_PAIR) {
       if (pair == _HashBase._DELETED_PAIR) {
@@ skipping 73 matching lines @@
       pair = _index[i];
     }
     return false;
   }
                                                 
   Iterator<K> get iterator =>
       new _CompactIterator<K>(this, _data, _usedData, -1, 1);
 
   Set<K> toSet() => new Set<K>()..addAll(this);  
 }