Remove Expect from core library.

tests/corelib/iterable_mapping_test.dart

 // Copyright (c) 2013, 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.addAll([11, 12, 13]);
   Set set2 = new Set();
 
   Iterable mapped = list1.map((x) => x + 1);
   Expect.listEquals([2, 3, 4], mapped.toList());
@@ skipping 25 matching lines @@
 
   mapped = set2.map((x) => x + 1);
   Expect.isFalse(mapped is List);
   Expect.listEquals([], mapped.toList());
 
   mapped = mapped.map((x) => x + 1);
   Expect.isFalse(mapped is List);
   Expect.listEquals([], mapped.toList());
 
 }