Make ClosedWorld a `forTesting` property of ResolutionWorldBuilder.

tests/compiler/dart2js/concrete_type_inference_test.dart

Index: tests/compiler/dart2js/concrete_type_inference_test.dart
diff --git a/tests/compiler/dart2js/concrete_type_inference_test.dart b/tests/compiler/dart2js/concrete_type_inference_test.dart
index 8c9cd6d67de9270a4d27f305e8845b059f7ae7d2..2efb5d7ecce349ae277d8be3159e5686b5b541de 100644
--- a/tests/compiler/dart2js/concrete_type_inference_test.dart
+++ b/tests/compiler/dart2js/concrete_type_inference_test.dart
@@ -20,21 +20,24 @@ void checkPrintType(String expression, checkType(closedWorld, type)) {
   asyncTest(() => compileAndFind('main() { print($expression); }', 'print',
           (compiler, printElement) {
         var parameter = printElement.functionSignature.requiredParameters.first;
-        checkType(compiler.closedWorld, _typeOf(compiler, parameter));
+        checkType(compiler.resolverWorld.closedWorldForTesting,
+            _typeOf(compiler, parameter));
       }));
 
   asyncTest(() =>
       compileAndFind('main() { var x = print; print($expression); }', 'print',
           (compiler, printElement) {
         var parameter = printElement.functionSignature.requiredParameters.first;
-        checkType(compiler.closedWorld, _typeOf(compiler, parameter));
+        checkType(compiler.resolverWorld.closedWorldForTesting,
+            _typeOf(compiler, parameter));
       }));
 
   asyncTest(() => compileAndFind(
           'main() { print($expression); print($expression); }', 'print',
           (compiler, printElement) {
         var parameter = printElement.functionSignature.requiredParameters.first;
-        checkType(compiler.closedWorld, _typeOf(compiler, parameter));
+        checkType(compiler.resolverWorld.closedWorldForTesting,
+            _typeOf(compiler, parameter));
       }));
 }
 
@@ -68,7 +71,7 @@ void testOptionalParameters() {
     var firstParameter = fiskElement.functionSignature.requiredParameters[0];
     var secondParameter = fiskElement.functionSignature.optionalParameters[0];
     var thirdParameter = fiskElement.functionSignature.optionalParameters[1];
-    var commonMasks = compiler.closedWorld.commonMasks;
+    var commonMasks = compiler.resolverWorld.closedWorldForTesting.commonMasks;
     Expect.identical(commonMasks.uint31Type, _typeOf(compiler, firstParameter));
     Expect.identical(commonMasks.nullType, _typeOf(compiler, secondParameter));
     Expect.identical(commonMasks.nullType, _typeOf(compiler, thirdParameter));