Make ClosedWorld a `forTesting` property of ResolutionWorldBuilder.

tests/compiler/dart2js/related_types.dart

Index: tests/compiler/dart2js/related_types.dart
diff --git a/tests/compiler/dart2js/related_types.dart b/tests/compiler/dart2js/related_types.dart
index af65613fab7fa0c823ed85e173c602d4cbf91c3c..61203ec979da5711ce345b0d4dc53f72d52a2aac 100644
--- a/tests/compiler/dart2js/related_types.dart
+++ b/tests/compiler/dart2js/related_types.dart
@@ -36,7 +36,6 @@ main(List<String> arguments) async {
 /// Check all loaded libraries in [compiler] for unrelated types.
 void checkRelatedTypes(Compiler compiler) {
   compiler.closeResolution();
-  compiler.closedWorld;
   for (LibraryElement library in compiler.libraryLoader.libraries) {
     checkLibraryElement(compiler, library);
   }
@@ -78,7 +77,7 @@ class RelatedTypesChecker extends TraversalVisitor<DartType, dynamic> {
       : this.resolvedAst = resolvedAst,
         super(resolvedAst.elements);
 
-  ClosedWorld get world => compiler.closedWorld;
+  ClosedWorld get world => compiler.resolverWorld.closedWorldForTesting;
 
   CoreClasses get coreClasses => compiler.coreClasses;