Move definition of Feature next to definitions of WorldImpact and Use

pkg/compiler/lib/src/universe/use.dart

Index: pkg/compiler/lib/src/universe/use.dart
diff --git a/pkg/compiler/lib/src/universe/use.dart b/pkg/compiler/lib/src/universe/use.dart
index e795f0234a5753e9fff6302d670a1f18540faa92..c1ab98f9d23eba35cc8343c53398b8b6104823ef 100644
--- a/pkg/compiler/lib/src/universe/use.dart
+++ b/pkg/compiler/lib/src/universe/use.dart
@@ -2,9 +2,18 @@
 // 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;