Further reduce use of Compiler.closedWorld.

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 78791cb3dbfae1bbbd2460b8bea81843ea44952e..609cac75e98b2b5e69c92ad2a05290cee8117764 100644
--- a/tests/compiler/dart2js/type_mask2_test.dart
+++ b/tests/compiler/dart2js/type_mask2_test.dart
@@ -104,7 +104,7 @@ Future testUnionTypeMaskFlatten() async {
       """,
       useMockCompiler: false);
 
-  ClosedWorld closedWorld = env.compiler.closedWorld;
+  ClosedWorld closedWorld = env.closedWorld;
 
   ClassElement Object_ = env.getElement("Object");
   ClassElement A = env.getElement("A");
@@ -213,12 +213,11 @@ Future testStringSubtypes() async {
       }
       """,
       useMockCompiler: false);
-  var closedWorld = env.compiler.closedWorld;
-  var backend = env.compiler.backend;
+  ClosedWorld closedWorld = env.closedWorld;
 
   ClassElement Object_ = env.getElement("Object");
   ClassElement String_ = env.getElement("String");
-  ClassElement JSString = backend.helpers.jsStringClass;
+  ClassElement JSString = closedWorld.backendClasses.stringImplementation;
 
   List<ClassElement> allClasses = <ClassElement>[Object_, String_];