Remove Expect from core library.

tests/corelib/iterable_first_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();
 
   Expect.equals(1, list1.first);
   Expect.equals(4, list2.first);
   Expect.throws(() => list3.first, (e) => e is StateError);
 
   Expect.isTrue(set1.contains(set1.first));
 
   Expect.throws(() => set2.first, (e) => e is StateError);
 }