Fix hints reported by analyzer.

pkg/compiler/lib/src/common/codegen.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 dart2js.common.codegen;
 
 import '../common.dart';
 import '../common/backend_api.dart' show Backend;
 import '../constants/values.dart' show ConstantValue;
 import '../elements/resolution_types.dart'
     show ResolutionDartType, ResolutionInterfaceType;
 import '../elements/elements.dart'
     show
         AstElement,
         ClassElement,
         Element,
         FunctionElement,
         LocalFunctionElement,
         ResolvedAst;
-import '../enqueue.dart' show Enqueuer;
 import '../universe/use.dart' show DynamicUse, StaticUse, TypeUse;
 import '../universe/world_impact.dart'
     show WorldImpact, WorldImpactBuilderImpl, WorldImpactVisitor;
 import '../util/util.dart' show Pair, Setlet;
 import 'work.dart' show WorkItem;
 
 class CodegenImpact extends WorldImpact {
   const CodegenImpact();
 
   Iterable<ConstantValue> get compileTimeConstants => const <ConstantValue>[];
@@ skipping 212 matching lines @@
       : this.resolvedAst = resolvedAst,
         super(resolvedAst.element);
 
   WorldImpact run() {
     registry = new CodegenRegistry(element);
     return backend.codegen(this);
   }
 
   String toString() => 'CodegenWorkItem(${resolvedAst.element})';
 }