Documentation to follow up on CLs 714843002 and 719443002.

dart/pkg/compiler/lib/src/universe/universe.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.
 
 library universe;
 
 import '../elements/elements.dart';
 import '../dart2jslib.dart';
 import '../dart_types.dart';
 import '../types/types.dart';
@@ skipping 730 matching lines @@
                        hashCode) {
     assert(mask != null);
     assert(asUntyped.mask == null);
   }
 
   static Map<Selector, Map<TypeMask, TypedSelector>> canonicalizedValues =
       new Map<Selector, Map<TypeMask, TypedSelector>>();
 
   factory TypedSelector(TypeMask mask, Selector selector, World world) {
     if (!world.hasClosedWorldAssumption) {
+      // TODO(johnniwinther): Improve use of TypedSelector in an open world.
       bool isNullable = mask.isNullable;
       mask = world.compiler.typesTask.dynamicType;
       if (isNullable) {
         mask = mask.nullable();
       }
     }
     // TODO(johnniwinther): Allow more TypeSelector kinds during resoluton.
     assert(world.isClosed || mask.isExact);
     if (selector.mask == mask) return selector;
     Selector untyped = selector.asUntyped;
@@ skipping 42 matching lines @@
 
   Selector extendIfReachesAll(Compiler compiler) {
     bool canReachAll = compiler.enabledInvokeOn
         && mask.needsNoSuchMethodHandling(this, compiler.world);
     return canReachAll
         ? new TypedSelector(
             compiler.typesTask.dynamicType, this, compiler.world)
         : this;
   }
 }