Move definition of Feature next to definitions of WorldImpact and Use

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 'dart:collection';
 
 import '../common.dart';
 import '../compiler.dart' show Compiler;
@@ skipping 84 matching lines @@
 }
 
 /// Strategy for computing the constraints on potential receivers of dynamic
 /// call sites.
 abstract class SelectorConstraintsStrategy {
   /// Create a [UniverseSelectorConstraints] to represent the global receiver
   /// constraints for dynamic call sites with [selector].
   UniverseSelectorConstraints createSelectorConstraints(Selector selector);
 }
 
+/// The [Universe] is an auxiliary class used in the process of computing the
+/// [ClassWorld]. The concepts here and in [ClassWorld] are very similar -- in
+/// the same way that the "universe expands" you can think of this as a mutable
+/// world that is expanding as we visit and discover parts of the program.
+/// TODO(sigmund): rename to "growing/expanding/mutable world"?
 class Universe {
   /// The set of all directly instantiated classes, that is, classes with a
   /// generative constructor that has been called directly and not only through
   /// a super-call.
   ///
   /// Invariant: Elements are declaration elements.
   // TODO(johnniwinther): [_directlyInstantiatedClasses] and
   // [_instantiatedTypes] sets should be merged.
   final Set<ClassElement> _directlyInstantiatedClasses =
       new Set<ClassElement>();
@@ skipping 274 matching lines @@
   // TODO(ahe): Replace this method with something that is O(1), for example,
   // by using a map.
   List<LocalFunctionElement> slowDirectlyNestedClosures(Element element) {
     // Return new list to guard against concurrent modifications.
     return new List<LocalFunctionElement>.from(
         allClosures.where((LocalFunctionElement closure) {
       return closure.executableContext == element;
     }));
   }
 }