dart2js cleanup: remove unused imports and variables

pkg/compiler/lib/src/ssa/optimize.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.
 
-import '../common/codegen.dart' show CodegenRegistry, CodegenWorkItem;
+import '../common/codegen.dart' show CodegenWorkItem;
 import '../common/tasks.dart' show CompilerTask;
 import '../compiler.dart' show Compiler;
 import '../constants/constant_system.dart';
 import '../constants/values.dart';
 import '../core_types.dart' show CoreClasses;
 import '../dart_types.dart';
 import '../elements/elements.dart';
 import '../js/js.dart' as js;
 import '../js_backend/backend_helpers.dart' show BackendHelpers;
 import '../js_backend/js_backend.dart';
 import '../native/native.dart' as native;
 import '../tree/tree.dart' as ast;
 import '../types/types.dart';
 import '../universe/selector.dart' show Selector;
 import '../universe/side_effects.dart' show SideEffects;
 import '../util/util.dart';
 import '../world.dart' show ClassWorld, World;
-
+import 'interceptor_simplifier.dart';
 import 'nodes.dart';
+import 'types.dart';
 import 'types_propagation.dart';
-import 'types.dart';
 import 'value_range_analyzer.dart';
 import 'value_set.dart';
-import 'interceptor_simplifier.dart';
 
 abstract class OptimizationPhase {
   String get name;
   void visitGraph(HGraph graph);
 }
 
 class SsaOptimizerTask extends CompilerTask {
   final JavaScriptBackend backend;
   SsaOptimizerTask(JavaScriptBackend backend)
       : this.backend = backend,
@@ skipping 2345 matching lines @@
 
     keyedValues.forEach((receiver, values) {
       result.keyedValues[receiver] =
           new Map<HInstruction, HInstruction>.from(values);
     });
 
     result.nonEscapingReceivers.addAll(nonEscapingReceivers);
     return result;
   }
 }