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

tests/compiler/dart2js/class_set_test.dart

Index: tests/compiler/dart2js/class_set_test.dart
diff --git a/tests/compiler/dart2js/class_set_test.dart b/tests/compiler/dart2js/class_set_test.dart
index 1d3b04f3a1159a5440bc35fcff1366505c66bc01..04da8001bf185e48926a64198683f9f7bb7be618 100644
--- a/tests/compiler/dart2js/class_set_test.dart
+++ b/tests/compiler/dart2js/class_set_test.dart
@@ -50,7 +50,7 @@ testIterators() async {
       }
       """,
       useMockCompiler: false);
-  World world = env.compiler.world;
+  World world = env.compiler.openWorld;
 
   ClassElement A = env.getElement("A");
   ClassElement B = env.getElement("B");
@@ -384,7 +384,7 @@ testForEach() async {
       }
       """,
       useMockCompiler: false);
-  World world = env.compiler.world;
+  World world = env.compiler.openWorld;
 
   ClassElement A = env.getElement("A");
   ClassElement B = env.getElement("B");