Move definition of Feature next to definitions of WorldImpact and Use

pkg/compiler/lib/src/world.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 dart2js.world;
 
 import 'closure.dart' show SynthesizedCallMethodElementX;
 import 'common/backend_api.dart' show Backend;
 import 'common.dart';
 import 'compiler.dart' show Compiler;
 import 'core_types.dart' show CoreClasses;
 import 'dart_types.dart';
 import 'elements/elements.dart'
     show
         ClassElement,
         Element,
         FunctionElement,
         MixinApplicationElement,
         TypedefElement,
         VariableElement;
 import 'ordered_typeset.dart';
 import 'types/types.dart' as ti;
 import 'universe/class_set.dart';
 import 'universe/function_set.dart' show FunctionSet;
 import 'universe/selector.dart' show Selector;
 import 'universe/side_effects.dart' show SideEffects;
 import 'util/util.dart' show Link;
 
+/// The [ClassWorld] represents the information known about a program when
+/// compiling with closed-world semantics.
+///
+/// Given the entrypoint of an application, we can track what's reachable from
+/// it, what functions are called, what classes are allocated, which native
+/// JavaScript types are touched, what language features are used, and so on.
+/// This precise knowledge about what's live in the program is later used in
+/// optimizations and other compiler decisions during code generation.
 abstract class ClassWorld {
   // TODO(johnniwinther): Refine this into a `BackendClasses` interface.
   Backend get backend;
 
   // TODO(johnniwinther): Remove the need for this getter.
   @deprecated
   Compiler get compiler;
 
   /// The [ClassElement] for the [Object] class defined in 'dart:core'.
   ClassElement get objectClass;
@@ skipping 788 matching lines @@
     // function expressions's element.
     // TODO(herhut): Generate classes for function expressions earlier.
     if (element is SynthesizedCallMethodElementX) {
       return getMightBePassedToApply(element.expression);
     }
     return functionsThatMightBePassedToApply.contains(element);
   }
 
   bool get hasClosedWorldAssumption => !compiler.options.hasIncrementalSupport;
 }