dart2js cleanup: remove unused imports and variables

pkg/compiler/lib/src/universe/call_structure.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.call_structure;
 
 import '../common.dart';
-import '../common/names.dart' show Identifiers, Names, Selectors;
+import '../common/names.dart' show Names;
 import '../elements/elements.dart';
 import '../tree/tree.dart';
 import '../util/util.dart';
-
 import 'selector.dart' show Selector;
 
 /// The structure of the arguments at a call-site.
 // TODO(johnniwinther): Should these be cached?
 // TODO(johnniwinther): Should isGetter/isSetter be part of the call structure
 // instead of the selector?
 class CallStructure {
   static const CallStructure NO_ARGS = const CallStructure.unnamed(0);
   static const CallStructure ONE_ARG = const CallStructure.unnamed(1);
   static const CallStructure TWO_ARGS = const CallStructure.unnamed(2);
@@ skipping 253 matching lines @@
       return first.compareTo(second);
     });
     return _orderedNamedArguments;
   }
 
   @override
   String structureToString() {
     return 'arity=$argumentCount, named=[${namedArguments.join(', ')}]';
   }
 }