Pass ClosedWorld directly to codegen tasks

pkg/compiler/lib/src/common/backend_api.dart

 // Copyright (c) 2012, 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.backend_api;
 
 import 'dart:async' show Future;
 
 import '../common.dart';
 import '../common/codegen.dart' show CodegenImpact;
@@ skipping 295 matching lines @@
   }
 
   /// Called after the queue is closed. [onQueueEmpty] may be called multiple
   /// times, but [onQueueClosed] is only called once.
   void onQueueClosed() {}
 
   /// Called when the compiler starts running the codegen enqueuer. The
   /// [WorldImpact] of enabled backend features is returned.
   WorldImpact onCodegenStart(ClosedWorld closedWorld) => const WorldImpact();
 
+  /// Called when code generation has been completed.
+  void onCodegenEnd() {}
+
   // Does this element belong in the output
   bool shouldOutput(Element element) => true;
 
   FunctionElement helperForBadMain() => null;
 
   FunctionElement helperForMissingMain() => null;
 
   FunctionElement helperForMainArity() => null;
 
   void forgetElement(Element element) {}
@@ skipping 90 matching lines @@
   ClassElement get asyncFutureImplementation;
   ClassElement get asyncStarStreamImplementation;
   ClassElement get indexableImplementation;
   ClassElement get mutableIndexableImplementation;
   ClassElement get indexingBehaviorImplementation;
 
   bool isDefaultEqualityImplementation(Element element);
   bool isInterceptorClass(ClassElement cls);
   bool isNative(Element element);
 }