Add Set.lookup method to get the instance in the set.

runtime/lib/collection_patch.dart

 // Copyright (c) 2013, 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.
 
 patch class HashMap<K, V> {
   /* patch */ factory HashMap({ bool equals(K key1, K key2),
                                 int hashCode(K key),
                                 bool isValidKey(potentialKey) }) {
     if (isValidKey == null) {
       if (hashCode == null) {
@@ skipping 551 matching lines @@
   bool contains(Object object) {
     int index = _hashCode(object) & (_buckets.length - 1);
     HashSetEntry entry = _buckets[index];
     while (entry != null) {
       if (_equals(entry.key, object)) return true;
       entry = entry.next;
     }
     return false;
   }
 
+  E lookup(Object object) {
+    int index = _hashCode(object) & (_buckets.length - 1);
+    HashSetEntry entry = _buckets[index];
+    while (entry != null) {
+      var key = entry.key;
+      if (_equals(key, object)) return key;
+      entry = entry.next;
+    }
+    return null;
+  }
+
   // Set.
 
   void add(E element) {
     int hashCode = _hashCode(element);
     int index = hashCode & (_buckets.length - 1);
     HashSetEntry entry = _buckets[index];
     while (entry != null) {
       if (_equals(entry.key, element)) return;
       entry = entry.next;
     }
@@ skipping 135 matching lines @@
   bool remove(Object element) {
     if (!_validKey(element)) return false;
     return super.remove(element);
   }
 
   bool contains(Object element) {
     if (!_validKey(element)) return false;
     return super.contains(element);
   }
 
+  E lookup(Object element) {
+    if (!_validKey(element)) return null;
+    return super.lookup(element);
+  }
+
   bool containsAll(Iterable<Object> elements) {
     for (Object element in elements) {
       if (!_validKey(element) || !this.contains(element)) return false;
     }
     return true;
   }
 
   void removeAll(Iterable<Object> elements) {
     for (Object element in elements) {
       if (_validKey(element)) {
@@ skipping 436 matching lines @@
 
   bool _equals(e1, e2) => _equality(e1, e2);
 
   int _hashCode(e) => _hasher(e);
 
   bool contains(Object o) {
     if (!_validKey(o)) return false;
     return super.contains(o);
   }
 
+  E lookup(Object o) {
+    if (!_validKey(o)) return null;
+    return super.lookup(o);
+  }
+
   bool remove(Object o) {
     if (!_validKey(o)) return false;
     return super.remove(o);
   }
 
   bool containsAll(Iterable<Object> elements) {
     for (Object element in elements) {
       if (!_validKey(element) || !this.contains(element)) return false;
     }
     return true;
@@ skipping 35 matching lines @@
       return false;
     }
     _LinkedHashSetEntry entry = _next;
     _current = entry.key;
     _next = entry._nextEntry;
     return true;
   }
 
   E get current => _current;
 }