Introduce OpenWorld.

tests/compiler/dart2js/union_type_test.dart

Index: tests/compiler/dart2js/union_type_test.dart
diff --git a/tests/compiler/dart2js/union_type_test.dart b/tests/compiler/dart2js/union_type_test.dart
index 6e722af77ecd36b020e4cda77a37165323fbe252..4306f61c4706ffacb1f53cd570c8df4bdf3ceb0b 100644
--- a/tests/compiler/dart2js/union_type_test.dart
+++ b/tests/compiler/dart2js/union_type_test.dart
@@ -22,8 +22,7 @@ main() {
       }
       """,
         useMockCompiler: false);
-    World world = env.compiler.closedWorld;
-    world.populate();
+    ClosedWorld world = env.compiler.openWorld.populate();
     FlatTypeMask mask1 = new FlatTypeMask.exact(env.getElement('A'));
     FlatTypeMask mask2 = new FlatTypeMask.exact(env.getElement('B'));
     UnionTypeMask union1 = mask1.nonNullable().union(mask2, world);