Fix hints reported by analyzer.

pkg/compiler/lib/src/universe/world_builder.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 universe;
 
 import 'dart:collection';
 
 import '../cache_strategy.dart';
 import '../common.dart';
 import '../common/backend_api.dart' show Backend;
 import '../common/names.dart' show Identifiers;
 import '../common/resolution.dart' show Resolution;
 import '../compiler.dart' show Compiler;
 import '../core_types.dart';
 import '../elements/resolution_types.dart';
 import '../elements/elements.dart';
 import '../elements/entities.dart';
 import '../universe/class_set.dart';
 import '../universe/function_set.dart' show FunctionSetBuilder;
 import '../util/enumset.dart';
 import '../util/util.dart';
 import '../world.dart' show World, ClosedWorld, ClosedWorldImpl, OpenWorld;
-import 'call_structure.dart' show CallStructure;
 import 'selector.dart' show Selector;
 import 'use.dart' show DynamicUse, DynamicUseKind, StaticUse, StaticUseKind;
 
 /// The known constraint on receiver for a dynamic call site.
 ///
 /// This can for instance be used to constrain this dynamic call to `foo` to
 /// 'receivers of the exact instance `Bar`':
 ///
 ///     class Bar {
 ///        void foo() {}
@@ skipping 1932 matching lines @@
   @override
   EnumSet<MemberUse> get _originalUse => MemberUses.ALL_STATIC;
 }
 
 void removeFromSet(Map<String, Set<_MemberUsage>> map, Element element) {
   Set<_MemberUsage> set = map[element.name];
   if (set == null) return;
   set.removeAll(
       set.where((_MemberUsage usage) => usage.entity == element).toList());
 }