Further reduce use of Compiler.closedWorld.

tests/compiler/dart2js/type_test_helper.dart

Index: tests/compiler/dart2js/type_test_helper.dart
diff --git a/tests/compiler/dart2js/type_test_helper.dart b/tests/compiler/dart2js/type_test_helper.dart
index cc0ae9093ed3c9868627c969682a8de905e24ebd..491ccc510697f6cb71ea20de936d76f75d63f389 100644
--- a/tests/compiler/dart2js/type_test_helper.dart
+++ b/tests/compiler/dart2js/type_test_helper.dart
@@ -14,6 +14,7 @@ import 'package:compiler/src/dart_types.dart';
 import 'package:compiler/src/compiler.dart' show Compiler;
 import 'package:compiler/src/elements/elements.dart'
     show Element, MemberElement, TypeDeclarationElement, ClassElement;
+import 'package:compiler/src/world.dart' show ClosedWorld;
 
 GenericType instantiate(
     TypeDeclarationElement element, List<DartType> arguments) {
@@ -139,4 +140,8 @@ class TypeEnvironment {
     return new FunctionType.synthesized(returnType, parameters,
         optionalParameters, namedParameterNames, namedParameterTypes);
   }
+
+  ClosedWorld get closedWorld {
+    return compiler.closedWorld;
+  }
 }