dart2js cleanup: remove unused imports and variables

pkg/compiler/lib/src/js_backend/js_backend.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 js_backend;
 
-import 'dart:async' show EventSink, Future;
+import 'dart:async' show Future;
 import 'dart:collection' show HashMap;
 
 import 'package:js_runtime/shared/embedded_names.dart' as embeddedNames;
 import 'package:js_runtime/shared/embedded_names.dart' show JsGetName;
 
 import '../closure.dart';
 import '../common.dart';
 import '../common/backend_api.dart'
-    show Backend, BackendSerialization, ImpactTransformer, ForeignResolver;
-import '../common/codegen.dart'
-    show CodegenImpact, CodegenRegistry, CodegenWorkItem;
+    show Backend, ImpactTransformer, ForeignResolver;
+import '../common/codegen.dart' show CodegenImpact, CodegenWorkItem;
 import '../common/names.dart' show Identifiers, Names, Selectors, Uris;
 import '../common/registry.dart' show EagerRegistry, Registry;
-import '../common/tasks.dart' show CompilerTask;
 import '../common/resolution.dart'
     show Feature, ListLiteralUse, MapLiteralUse, Resolution, ResolutionImpact;
+import '../common/tasks.dart' show CompilerTask;
 import '../common/work.dart' show ItemCompilationContext;
+import '../compile_time_constants.dart';
 import '../compiler.dart' show Compiler;
-import '../compile_time_constants.dart';
 import '../constants/constant_system.dart';
 import '../constants/expressions.dart';
 import '../constants/values.dart';
 import '../core_types.dart' show CoreClasses, CoreTypes;
 import '../dart_types.dart';
 import '../deferred_load.dart' show DeferredLoadTask;
 import '../diagnostics/invariant.dart' show DEBUG_MODE;
 import '../dump_info.dart' show DumpInfoTask;
 import '../elements/elements.dart';
 import '../elements/visitor.dart' show BaseElementVisitor;
 import '../enqueue.dart' show Enqueuer, ResolutionEnqueuer;
 import '../io/code_output.dart';
+import '../io/position_information.dart' show PositionSourceInformationStrategy;
 import '../io/source_information.dart' show SourceInformationStrategy;
-import '../io/position_information.dart' show PositionSourceInformationStrategy;
 import '../io/start_end_information.dart'
     show StartEndSourceInformationStrategy;
 import '../js/js.dart' as jsAst;
 import '../js/js.dart' show js;
 import '../js/js_source_mapping.dart' show JavaScriptSourceInformationStrategy;
 import '../js/rewrite_async.dart';
 import '../js_emitter/js_emitter.dart'
-    show
-        CodeEmitterTask,
-        Emitter,
-        MetadataCollector,
-        Placeholder,
-        USE_LAZY_EMITTER;
+    show CodeEmitterTask, MetadataCollector, Placeholder;
 import '../library_loader.dart' show LibraryLoader, LoadedLibraries;
 import '../native/native.dart' as native;
 import '../resolution/tree_elements.dart' show TreeElements;
 import '../ssa/builder.dart' show SsaFunctionCompiler;
+import '../ssa/codegen.dart' show SsaCodeGenerator;
 import '../ssa/nodes.dart' show HTypeConversion, HInstruction;
-import '../ssa/codegen.dart' show SsaCodeGenerator;
 import '../tree/tree.dart';
 import '../types/types.dart';
 import '../universe/call_structure.dart' show CallStructure;
 import '../universe/selector.dart' show Selector, SelectorKind;
 import '../universe/universe.dart';
 import '../universe/use.dart'
     show DynamicUse, StaticUse, StaticUseKind, TypeUse, TypeUseKind;
 import '../universe/world_impact.dart'
     show
         ImpactStrategy,
         ImpactUseCase,
         TransformedWorldImpact,
         WorldImpact,
         WorldImpactVisitor;
 import '../util/characters.dart';
 import '../util/util.dart';
 import '../world.dart' show ClassWorld;
-
 import 'backend_helpers.dart';
 import 'backend_impact.dart';
 import 'backend_serialization.dart' show JavaScriptBackendSerialization;
 import 'codegen/task.dart';
 import 'constant_system_javascript.dart';
-import 'native_data.dart' show NativeData;
 import 'js_interop_analysis.dart' show JsInteropAnalysis;
 import 'lookup_map_analysis.dart' show LookupMapAnalysis;
+import 'native_data.dart' show NativeData;
 import 'patch_resolver.dart';
 
 part 'backend.dart';
 part 'checked_mode_helpers.dart';
 part 'constant_emitter.dart';
 part 'constant_handler_javascript.dart';
 part 'custom_elements_analysis.dart';
+part 'field_naming_mixin.dart';
 part 'frequency_namer.dart';
-part 'field_naming_mixin.dart';
 part 'minify_namer.dart';
 part 'namer.dart';
 part 'namer_names.dart';
 part 'no_such_method_registry.dart';
 part 'runtime_types.dart';
 part 'type_variable_handler.dart';