Move definition of Feature next to definitions of WorldImpact and Use

pkg/compiler/lib/src/world.dart

Index: pkg/compiler/lib/src/world.dart
diff --git a/pkg/compiler/lib/src/world.dart b/pkg/compiler/lib/src/world.dart
index 96ea689770e93ce3a2b5df75b13eef6490d02c20..99aca9f29bcf87d8a42f0a0ba9b0b2ab3ab6d625 100644
--- a/pkg/compiler/lib/src/world.dart
+++ b/pkg/compiler/lib/src/world.dart
@@ -26,6 +26,14 @@ 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;