Extract ClosedWorldImpl from WorldImpl.

pkg/compiler/lib/src/use_unused_api.dart

Index: pkg/compiler/lib/src/use_unused_api.dart
diff --git a/pkg/compiler/lib/src/use_unused_api.dart b/pkg/compiler/lib/src/use_unused_api.dart
index da1cbdff90ac04e03361e2173ab60b418954d4aa..cff74c49ad7041b8c61211ad4e7d72611d6af702 100644
--- a/pkg/compiler/lib/src/use_unused_api.dart
+++ b/pkg/compiler/lib/src/use_unused_api.dart
@@ -247,15 +247,15 @@ useIo([io.LineColumnMap map, io.LineColumnProvider provider]) {
 usedByTests() {
   // TODO(ahe): We should try to avoid including API used only for tests. In
   // most cases, such API can be moved to a test library.
-  WorldImpl world = null;
+  ClosedWorldImpl closedWorld = null;
   type_graph_inferrer.TypeGraphInferrer typeGraphInferrer = null;
   source_file_provider.SourceFileProvider sourceFileProvider = null;
   sourceFileProvider.getSourceFile(null);
-  world.hasAnyUserDefinedGetter(null, null);
-  world.subclassesOf(null);
-  world.getClassHierarchyNode(null);
-  world.getClassSet(null);
-  world.haveAnyCommonSubtypes(null, null);
+  closedWorld.hasAnyUserDefinedGetter(null, null);
+  closedWorld.subclassesOf(null);
+  closedWorld.getClassHierarchyNode(null);
+  closedWorld.getClassSet(null);
+  closedWorld.haveAnyCommonSubtypes(null, null);
   typeGraphInferrer.getCallersOf(null);
   dart_types.Types.sorted(null);
   new dart_types.Types(null).copy(null);