Move towards using WorldImpact for codegen

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 b908e9f9144943a4f8be0b8bb128d51af4377062..e313c71521bac0cf27d1beaf2d66eb6f971f8939 100644
--- a/tests/compiler/dart2js/type_combination_test.dart
+++ b/tests/compiler/dart2js/type_combination_test.dart
@@ -739,7 +739,7 @@ void main() {
     """);
     JavaScriptBackend backend = compiler.backend;
     BackendHelpers helpers = backend.helpers;
-    ClosedWorld world = compiler.closedWorld;
+    ClosedWorld world = compiler.openWorld.closeWorld();
     helpers.interceptorsLibrary.forEachLocalMember((element) {
       if (element.isClass) {
         element.ensureResolved(compiler.resolution);
@@ -764,7 +764,7 @@ void main() {
         patternImplClass.rawType,
         compiler.enqueuer.resolution,
         compiler.globalDependencies);
-    compiler.openWorld.populate();
+    compiler.openWorld.closeWorld();
 
     // Grab hold of a supertype for String so we can produce potential
     // string types.