Migrate test block 11 to Dart 2.0.

tests/corelib/iterable_single_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 "package:expect/expect.dart";
-
-main() {
-  List<int> list1a = <int>[1];
-  List<int> list1b = <int>[1, 2, 3];
-  List<int> list1c = <int>[];
-  List<int> list2a = const <int>[5];
-  List<int> list2b = const <int>[4, 5];
-  List<int> list2c = const <int>[];
-  Set<int> set1 = new Set<int>();
-  set1..add(22);
-  Set set2 = new Set();
-  set2..add(11)..add(12)..add(13);
-  Set set3 = new Set();
-
-  Expect.equals(1, list1a.single);
-  Expect.throws(() => list1b.single, (e) => e is StateError);
-  Expect.throws(() => list1c.single, (e) => e is StateError);
-
-  Expect.equals(5, list2a.single);
-  Expect.throws(() => list2b.single, (e) => e is StateError);
-  Expect.throws(() => list2c.single, (e) => e is StateError);
-
-  Expect.equals(22, set1.single);
-  Expect.throws(() => set2.single, (e) => e is StateError);
-  Expect.throws(() => set3.single, (e) => e is StateError);
-}