Fix hints reported by analyzer.

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

 // Copyright (c) 2015, 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.universe.world_impact;
 
-import '../elements/elements.dart' show Element;
 import '../util/util.dart' show Setlet;
 import 'use.dart' show DynamicUse, StaticUse, TypeUse;
 
 /// Describes how an element (e.g. a method) impacts the closed-world
 /// semantics of a program.
 ///
 /// A [WorldImpact] contains information about how a program element affects our
 /// understanding of what's live in a program. For example, it can indicate
 /// that a method uses a certain feature, or allocates a specific type.
 ///
@@ skipping 294 matching lines @@
     }
   }
 
   @override
   void visitTypeUse(TypeUse use) {
     if (_visitTypeUse != null) {
       _visitTypeUse(use);
     }
   }
 }