Remove one more unused import

pkg/kernel/lib/transformations/closure_conversion.dart

 // Copyright (c) 2016, 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 kernel.transformations.closure_conversion;
 
-import '../ast.dart' show Class, Program, Library;
+import '../ast.dart' show Program, Library;
 
 import '../core_types.dart' show CoreTypes;
 
 import 'closure/converter.dart' show ClosureConverter;
 
 import 'closure/info.dart' show ClosureInfo;
 
 import 'closure/invalidate_closures.dart';
 
 Program transformProgram(CoreTypes coreTypes, Program program) {
@@ skipping 13 matching lines @@
 
   var convert = new ClosureConverter(coreTypes, info);
   for (int i = 0; i < libraries.length; i++) {
     libraries[i] = convert.visitLibrary(libraries[i]);
   }
   var invalidator = new InvalidateClosures();
   for (int i = 0; i < libraries.length; i++) {
     invalidator.visitLibrary(libraries[i]);
   }
 }