Make ClosedWorld a `forTesting` property of ResolutionWorldBuilder.

tests/compiler/dart2js/related_types.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library related_types;
 
 import 'package:compiler/src/commandline_options.dart';
 import 'package:compiler/src/compiler.dart';
 import 'package:compiler/src/core_types.dart';
 import 'package:compiler/src/dart_types.dart';
@@ skipping 18 matching lines @@
       checkRelatedTypes(result.compiler);
     }
   } else {
     print('Usage dart related_types.dart <entry-point>');
   }
 }
 
 /// 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);
   }
 }
 
 /// Check [library] for unrelated types.
 void checkLibraryElement(Compiler compiler, LibraryElement library) {
   library.forEachLocalMember((Element element) {
     if (element.isClass) {
       ClassElement cls = element;
@@ skipping 21 matching lines @@
 }
 
 class RelatedTypesChecker extends TraversalVisitor<DartType, dynamic> {
   final Compiler compiler;
   final ResolvedAst resolvedAst;
 
   RelatedTypesChecker(this.compiler, ResolvedAst resolvedAst)
       : this.resolvedAst = resolvedAst,
         super(resolvedAst.elements);
 
-  ClosedWorld get world => compiler.closedWorld;
+  ClosedWorld get world => compiler.resolverWorld.closedWorldForTesting;
 
   CoreClasses get coreClasses => compiler.coreClasses;
 
   CoreTypes get coreTypes => compiler.coreTypes;
 
   DiagnosticReporter get reporter => compiler.reporter;
 
   InterfaceType get thisType => resolvedAst.element.enclosingClass.thisType;
 
   /// Returns `true` if there exists no common subtype of [left] and [right].
@@ skipping 320 matching lines @@
   ClassElement findClass(DartType type) => type.accept(this, null);
 
   @override
   ClassElement visitType(DartType type, _) => null;
 
   @override
   ClassElement visitInterfaceType(InterfaceType type, _) {
     return type.element;
   }
 }