fix unsound cast failures in tests

tests/corelib_strong/iterable_empty_test.dart

 // Copyright (c) 2013, 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";
 
 main() {
-  testEmpty(name, it, [depth = 2]) {
+  testEmpty(name, Iterable<int> it, [depth = 2]) {
     Expect.isTrue(it.isEmpty, name);
     Expect.isFalse(it.isNotEmpty, name);
     Expect.equals(0, it.length, name);
     Expect.isFalse(it.contains(null), name);
     Expect.isFalse(it.any((x) => true), name);
     Expect.isTrue(it.every((x) => false), name);
     Expect.throws(() => it.first, (e) => e is StateError, name);
     Expect.throws(() => it.last, (e) => e is StateError, name);
     Expect.throws(() => it.single, (e) => e is StateError, name);
     Expect.throws(() => it.elementAt(0), (e) => e is RangeError, name);
@@ skipping 43 matching lines @@
   }
 
   test(name, it) {
     testEmpty(name, it);
     testType(name, it);
   }
 
   test("const", const Iterable<int>.empty());
   test("new", new Iterable<int>.empty());
 }