Add corelib tests

test/codegen/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);
+}