Remove deprecated Expect from the libraries.

tests/corelib/iterable_first_where_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> list1 = <int>[1, 2, 3];
   List<int> list2 = const <int>[4, 5];
   List<String> list3 = <String>[];
   Set<int> set1 = new Set<int>();
   set1..add(11)
       ..add(12)
       ..add(13);
   Set set2 = new Set();
 
@@ skipping 22 matching lines @@
   Expect.isTrue(odd == 11 || odd == 13);
   Expect.throws(() => set1.firstWhere((x) => false), (e) => e is StateError);
   Expect.equals(null, set1.firstWhere((x) => false, orElse: () => null));
   Expect.equals(499, set1.firstWhere((x) => false, orElse: () => 499));
 
   Expect.throws(() => set2.firstWhere((x) => false), (e) => e is StateError);
   Expect.throws(() => set2.firstWhere((x) => true), (e) => e is StateError);
   Expect.equals(null, set2.firstWhere((x) => true, orElse: () => null));
   Expect.equals(499, set2.firstWhere((x) => false, orElse: () => 499));
 }