Make ClosedWorld a `forTesting` property of ResolutionWorldBuilder.

tests/compiler/dart2js/expect_annotations_test.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.
 
 import 'package:expect/expect.dart';
 import 'package:async_helper/async_helper.dart';
 import 'package:compiler/src/compiler.dart';
 import 'package:compiler/src/elements/elements.dart';
 import 'package:compiler/src/js_backend/js_backend.dart';
 import 'package:compiler/src/types/types.dart';
@@ skipping 34 matching lines @@
   print(methodAssumeDynamicTrustTypeAnnotations(null));
 }
 """
 };
 
 main() {
   asyncTest(() async {
     CompilationResult result =
         await runCompiler(memorySourceFiles: MEMORY_SOURCE_FILES);
     Compiler compiler = result.compiler;
-    ClosedWorld closedWorld = compiler.closedWorld;
+    ClosedWorld closedWorld = compiler.resolverWorld.closedWorldForTesting;
     Expect.isFalse(compiler.compilationFailed, 'Unsuccessful compilation');
     JavaScriptBackend backend = compiler.backend;
     Expect.isNotNull(backend.annotations.expectNoInlineClass,
         'NoInlineClass is unresolved.');
     Expect.isNotNull(backend.annotations.expectTrustTypeAnnotationsClass,
         'TrustTypeAnnotations is unresolved.');
     Expect.isNotNull(backend.annotations.expectAssumeDynamicClass,
         'AssumeDynamicClass is unresolved.');
 
     void testTypeMatch(FunctionElement function, TypeMask expectedParameterType,
@@ skipping 52 matching lines @@
         expectNoInline: true,
         expectTrustTypeAnnotations: true,
         expectedParameterType: jsStringType,
         expectedReturnType: jsIntType);
     test('methodAssumeDynamicTrustTypeAnnotations',
         expectAssumeDynamic: true,
         expectTrustTypeAnnotations: true,
         expectedParameterType: coreStringType);
   });
 }