Split World usage into open, inference, and closed world.

tests/compiler/dart2js/type_mask2_test.dart

Index: tests/compiler/dart2js/type_mask2_test.dart
diff --git a/tests/compiler/dart2js/type_mask2_test.dart b/tests/compiler/dart2js/type_mask2_test.dart
index 03db5802e1599e2bf63fb558f39237b427076cf7..ba6f2db7d8633f59e33b9c7ff1bde0f2afae60a4 100644
--- a/tests/compiler/dart2js/type_mask2_test.dart
+++ b/tests/compiler/dart2js/type_mask2_test.dart
@@ -103,7 +103,7 @@ Future testUnionTypeMaskFlatten() async  {
       """,
       useMockCompiler: false);
 
-  ClassWorld classWorld = env.compiler.world;
+  ClassWorld classWorld = env.compiler.closedWorld;
 
   ClassElement Object_ = env.getElement("Object");
   ClassElement A = env.getElement("A");
@@ -224,7 +224,7 @@ Future testStringSubtypes() async {
       }
       """,
       useMockCompiler: false);
-    var classWorld = env.compiler.world;
+    var classWorld = env.compiler.closedWorld;
     var backend = env.compiler.backend;
 
     ClassElement Object_ = env.getElement("Object");