Remove stale comment in ImmutableList implementation

runtime/lib/immutable_map.dart

 // Copyright (c) 2012, 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.
 // Immutable map class for compiler generated map literals.
 
 class ImmutableMap<K, V> implements Map<K, V> {
   final _ImmutableList _kvPairs;
 
   const ImmutableMap._create(_ImmutableList keyValuePairs)
       : _kvPairs = keyValuePairs;
 
 
   V operator [](Object key) {
-    // TODO(hausner): Since the keys are sorted, we could do a binary
-    // search. But is it worth it?
+    // To preserve the key-value order of the map literal, the keys are
+    // not sorted. Need to do linear search or implement an additional
+    // lookup table.
     for (int i = 0; i < _kvPairs.length - 1; i += 2) {
       if (key == _kvPairs[i]) {
         return _kvPairs[i+1];
       }
     }
     return null;
   }
 
   bool get isEmpty {
     return _kvPairs.length == 0;
@@ skipping 118 matching lines @@
       _current = _map._kvPairs[newIndex * 2 + 1];
       return true;
     }
     _current = null;
     _index = _map.length;
     return false;
   }
 
   E get current => _current;
 }