Move definition of Feature next to definitions of WorldImpact and Use

pkg/compiler/lib/src/universe/use.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.
 
-/// This library defined `uses`. A `use` is a single impact of the world, for
-/// instance an invocation of a top level function or a call to the `foo()`
-/// method on an unknown class.
+/// This library defines individual world impacts.
+///
+/// We call these building blocks `uses`. Each `use` is a single impact of the
+/// world. Some example uses are:
+///
+///  * an invocation of a top level function
+///  * a call to the `foo()` method on an unknown class.
+///  * an instantiation of class T
+///
+/// The different compiler stages combine these uses into `WorldImpact` objects,
+/// which are later used to construct a closed-world understanding of the
+/// program.
 library dart2js.universe.use;
 
 import '../closure.dart' show BoxFieldElement;
 import '../common.dart';
 import '../dart_types.dart';
 import '../elements/elements.dart';
 import '../util/util.dart' show Hashing;
 import '../world.dart' show ClassWorld;
 import 'call_structure.dart' show CallStructure;
 import 'selector.dart' show Selector;
@@ skipping 283 matching lines @@
   }
 
   bool operator ==(other) {
     if (identical(this, other)) return true;
     if (other is! TypeUse) return false;
     return type == other.type && kind == other.kind;
   }
 
   String toString() => 'TypeUse($type,$kind)';
 }