Decouple WorkItem from Compiler

pkg/compiler/lib/src/serialization/impact_serialization.dart

Index: pkg/compiler/lib/src/serialization/impact_serialization.dart
diff --git a/pkg/compiler/lib/src/serialization/impact_serialization.dart b/pkg/compiler/lib/src/serialization/impact_serialization.dart
index c6ce2581275ce6bca17d520c04cf3194636f33c7..90c1570c8cb97398e1d098d5bb65bf0b65dedf4e 100644
--- a/pkg/compiler/lib/src/serialization/impact_serialization.dart
+++ b/pkg/compiler/lib/src/serialization/impact_serialization.dart
@@ -116,6 +116,9 @@ class DeserializedResolutionImpact extends WorldImpact
       this.nativeData: const <dynamic>[]})
       : this._features = features;
 
+  @override
+  bool get isEmpty => false;
+
   Iterable<Feature> get features {
     return _features != null
         ? _features.iterable(Feature.values)