Decouple WorkItem from Compiler

tests/compiler/dart2js/type_combination_test.dart

Index: tests/compiler/dart2js/type_combination_test.dart
diff --git a/tests/compiler/dart2js/type_combination_test.dart b/tests/compiler/dart2js/type_combination_test.dart
index 875f276288b932a3fc9f39e3b04d5a6a39be6a61..bde511b5c6c5cb3f84dce22208785ab37c47a456 100644
--- a/tests/compiler/dart2js/type_combination_test.dart
+++ b/tests/compiler/dart2js/type_combination_test.dart
@@ -8,6 +8,8 @@ import 'package:compiler/src/js_backend/backend_helpers.dart';
 import 'package:compiler/src/js_backend/js_backend.dart';
 import 'package:compiler/src/types/types.dart';
 import 'package:compiler/src/world.dart';
+import 'package:compiler/src/universe/use.dart';
+import 'package:compiler/src/universe/world_impact.dart';
 import 'compiler_helper.dart';
 import 'type_mask_test_helper.dart';
 
@@ -737,21 +739,24 @@ void main() {
     JavaScriptBackend backend = compiler.backend;
     BackendHelpers helpers = backend.helpers;
     ClosedWorld world = compiler.openWorld.closeWorld(compiler.reporter);
+    WorldImpactBuilderImpl impactBuilder = new WorldImpactBuilderImpl();
     helpers.interceptorsLibrary.forEachLocalMember((element) {
       if (element.isClass) {
         element.ensureResolved(compiler.resolution);
-        compiler.enqueuer.resolution.registerInstantiatedType(element.rawType);
+        impactBuilder
+            .registerTypeUse(new TypeUse.instantiation(element.rawType));
       }
     });
     ClassElement patternImplClass = compiler.mainApp.find('PatternImpl');
     patternImplClass.ensureResolved(compiler.resolution);
 
-    compiler.enqueuer.resolution
-        .registerInstantiatedType(compiler.coreTypes.mapType());
-    compiler.enqueuer.resolution
-        .registerInstantiatedType(compiler.coreTypes.functionType);
-    compiler.enqueuer.resolution
-        .registerInstantiatedType(patternImplClass.rawType);
+    impactBuilder.registerTypeUse(
+        new TypeUse.instantiation(compiler.coreTypes.mapType()));
+    impactBuilder.registerTypeUse(
+        new TypeUse.instantiation(compiler.coreTypes.functionType));
+    impactBuilder
+        .registerTypeUse(new TypeUse.instantiation(patternImplClass.rawType));
+    compiler.enqueuer.resolution.applyImpact(impactBuilder);
     compiler.openWorld.closeWorld(compiler.reporter);
 
     // Grab hold of a supertype for String so we can produce potential