dart2js cleanup: remove unused imports and variables

pkg/compiler/lib/src/ssa/invoke_dynamic_specializers.dart

 // Copyright (c) 2013, 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.
 
 import '../compiler.dart' show Compiler;
 import '../constants/constant_system.dart';
 import '../constants/values.dart';
 import '../elements/elements.dart';
 import '../js_backend/js_backend.dart';
 import '../types/types.dart';
+import '../universe/call_structure.dart';
 import '../universe/selector.dart';
-import '../universe/call_structure.dart';
-import '../world.dart' show ClassWorld, World;
-
+import '../world.dart' show World;
 import 'nodes.dart';
 import 'types.dart';
 
 /**
  * [InvokeDynamicSpecializer] and its subclasses are helpers to
  * optimize intercepted dynamic calls. It knows what input types
  * would be beneficial for performance, and how to change a invoke
  * dynamic to a builtin instruction (e.g. HIndex, HBitNot).
  */
 class InvokeDynamicSpecializer {
@@ skipping 728 matching lines @@
     return constantSystem.codeUnitAt;
   }
 
   HInstruction tryConvertToBuiltin(
       HInvokeDynamic instruction, Compiler compiler) {
     // TODO(sra): Implement a builtin HCodeUnitAt instruction and the same index
     // bounds checking optimizations as for HIndex.
     return null;
   }
 }