Rename XMatching to XWhere.

tests/corelib/iterable_single_matching_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.
-
-main() {
-  List<int> list1 = <int>[1, 2, 3];
-  List<int> list2 = const <int>[4, 5, 6];
-  List<String> list3 = <String>[];
-  Set<int> set1 = new Set<int>();
-  set1..add(11)
-      ..add(12)
-      ..add(13);
-  Set set2 = new Set();
-
-  Expect.equals(2, list1.singleMatching((x) => x.isEven));
-  Expect.equals(3, list1.singleMatching((x) => x == 3));
-  Expect.throws(() => list1.singleMatching((x) => x.isOdd),
-                (e) => e is StateError);
-
-  Expect.equals(6, list2.singleMatching((x) => x == 6));
-  Expect.equals(5, list2.singleMatching((x) => x.isOdd));
-  Expect.throws(() => list2.singleMatching((x) => x.isEven),
-                (e) => e is StateError);
-
-  Expect.throws(() => list3.singleMatching((x) => x == 0),
-                (e) => e is StateError);
-
-  Expect.equals(12, set1.singleMatching((x) => x.isEven));
-  Expect.equals(11, set1.singleMatching((x) => x == 11));
-  Expect.throws(() => set1.singleMatching((x) => x.isOdd));
-
-  Expect.throws(() => set2.singleMatching((x) => true), (e) => e is StateError);
-}