Move towards using WorldImpact for codegen

tests/compiler/dart2js/union_type_test.dart

 // Copyright (c) 2013, 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.
 
 import "package:async_helper/async_helper.dart";
 import "package:expect/expect.dart";
 import "package:compiler/src/types/types.dart";
 import "package:compiler/src/world.dart";
 import 'type_test_helper.dart';
 
 main() {
   asyncTest(() async {
     TypeEnvironment env = await TypeEnvironment.create(
         r"""
       class A {}
       class B {}
       """,
         mainSource: r"""
       main() {
         new A();
         new B();
       }
       """,
         useMockCompiler: false);
-    ClosedWorld world = env.compiler.openWorld.populate();
+    ClosedWorld world = env.compiler.openWorld.closeWorld();
     FlatTypeMask mask1 = new FlatTypeMask.exact(env.getElement('A'));
     FlatTypeMask mask2 = new FlatTypeMask.exact(env.getElement('B'));
     UnionTypeMask union1 = mask1.nonNullable().union(mask2, world);
     UnionTypeMask union2 = mask2.nonNullable().union(mask1, world);
     Expect.equals(union1, union2);
   });
 }