Decouple WorkItem from Compiler

pkg/compiler/lib/src/serialization/impact_serialization.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 dart2js.serialization.impact;
 
 import '../common/resolution.dart';
 import '../constants/expressions.dart';
 import '../dart_types.dart';
 import '../elements/elements.dart';
@@ skipping 98 matching lines @@
       this.constantLiterals: const <ConstantExpression>[],
       this.dynamicUses: const <DynamicUse>[],
       EnumSet<Feature> features,
       this.listLiterals: const <ListLiteralUse>[],
       this.mapLiterals: const <MapLiteralUse>[],
       this.staticUses: const <StaticUse>[],
       this.typeUses: const <TypeUse>[],
       this.nativeData: const <dynamic>[]})
       : this._features = features;
 
+  @override
+  bool get isEmpty => false;
+
   Iterable<Feature> get features {
     return _features != null
         ? _features.iterable(Feature.values)
         : const <Feature>[];
   }
 }
 
 class ImpactDeserializer {
   /// Deserializes a [WorldImpact] from [objectDecoder].
   static ResolutionImpact deserializeImpact(Element element,
@@ skipping 84 matching lines @@
         constantLiterals: constantLiterals,
         dynamicUses: dynamicUses,
         features: features,
         listLiterals: listLiterals,
         mapLiterals: mapLiterals,
         staticUses: staticUses,
         typeUses: typeUses,
         nativeData: nativeData);
   }
 }