Decouple WorkItem from Compiler

pkg/compiler/lib/src/universe/world_impact.dart

Index: pkg/compiler/lib/src/universe/world_impact.dart
diff --git a/pkg/compiler/lib/src/universe/world_impact.dart b/pkg/compiler/lib/src/universe/world_impact.dart
index 90120575b3fb147615ecce641fc69da545b4aa5a..2a1afb812b1bcae59b38f912e87d38558a3c2665 100644
--- a/pkg/compiler/lib/src/universe/world_impact.dart
+++ b/pkg/compiler/lib/src/universe/world_impact.dart
@@ -33,6 +33,8 @@ class WorldImpact {
 
   Iterable<TypeUse> get typeUses => const <TypeUse>[];
 
+  bool get isEmpty => true;
+
   void apply(WorldImpactVisitor visitor) {
     staticUses.forEach(visitor.visitStaticUse);
     dynamicUses.forEach(visitor.visitDynamicUse);
@@ -74,6 +76,18 @@ class WorldImpactBuilderImpl extends WorldImpact implements WorldImpactBuilder {
   Set<StaticUse> _staticUses;
   Set<TypeUse> _typeUses;
 
+  @override
+  bool get isEmpty =>
+      _dynamicUses == null && _staticUses == null && _typeUses == null;
+
+  /// Copy uses in [impact] to this impact builder.
+  void addImpact(WorldImpact impact) {
+    if (impact.isEmpty) return;
+    impact.dynamicUses.forEach(registerDynamicUse);
+    impact.staticUses.forEach(registerStaticUse);
+    impact.typeUses.forEach(registerTypeUse);
+  }
+
   void registerDynamicUse(DynamicUse dynamicUse) {
     assert(dynamicUse != null);
     if (_dynamicUses == null) {
@@ -174,6 +188,14 @@ class TransformedWorldImpact implements WorldImpact, WorldImpactBuilder {
   TransformedWorldImpact(this.worldImpact);
 
   @override
+  bool get isEmpty {
+    return worldImpact.isEmpty &&
+        _staticUses == null &&
+        _typeUses == null &&
+        _dynamicUses == null;
+  }
+
+  @override
   Iterable<DynamicUse> get dynamicUses {
     return _dynamicUses != null ? _dynamicUses : worldImpact.dynamicUses;
   }