Update dart2js unit tests to new library API.

dart/tests/compiler/dart2js/mirrors_test.dart

 // Copyright (c) 2012, 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 '../../../sdk/lib/_internal/compiler/implementation/mirrors/mirrors.dart';
 import '../../../sdk/lib/_internal/compiler/implementation/mirrors/mirrors_util.dart';
 
 import 'dart:io';
 
 int count(Iterable iterable) {
@@ skipping 268 matching lines @@
   Expect.isTrue(metadata8 is InstanceMirror);
   Expect.isFalse(metadata8.hasReflectee);
   Expect.throws(() => metadata8.reflectee, (_) => true);
   Expect.equals(metadataType.originalDeclaration, metadata8.type);
   metadata8.getField('data').then((InstanceMirror data) {
     Expect.isTrue(data is MapInstanceMirror);
     Expect.isFalse(data.hasReflectee);
     Expect.throws(() => data.reflectee, (_) => true);
     MapInstanceMirror mapData = data;
     Expect.equals(1, mapData.length);
-    Expect.stringEquals('foo', mapData.keys.iterator().next());
+    var it = mapData.keys.iterator;
+    Expect.isTrue(it.moveNext());
+    Expect.stringEquals('foo', it.current);
     mapData['foo'].then((InstanceMirror element) {
       Expect.isNotNull(element);
       Expect.isTrue(element.hasReflectee);
       Expect.stringEquals("Foo", element.reflectee);
     });
     Expect.isNull(mapData['bar']);
   });
 
   // @metadata
   var metadata9 = metadata[9];
@@ skipping 651 matching lines @@
   Expect.isTrue(privateFactoryConstructor.isPrivate);
   Expect.isFalse(privateFactoryConstructor.isConstConstructor);
   Expect.isFalse(privateFactoryConstructor.isRedirectingConstructor);
   Expect.isFalse(privateFactoryConstructor.isGenerativeConstructor);
   Expect.isTrue(privateFactoryConstructor.isFactoryConstructor);
 
   var metadata = privateClass.metadata;
   Expect.isNotNull(metadata);
   Expect.equals(0, metadata.length);
 }