Revert "Remove Expect from core library."

tests/corelib/list_removeat_test.dart

 // Copyright (c) 2011, 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";
-
 void main() {
   // Normal modifiable list.
   var l1 = [0, 1, 2, 3, 4];
 
   bool checkedMode = false;
   assert(checkedMode = true);
 
   // Index must be integer and in range.
   Expect.throws(() { l1.removeAt(-1); },
                 (e) => e is RangeError,
@@ skipping 38 matching lines @@
   Expect.throws(() { l3.removeAt(2); },
                 (e) => e is UnsupportedError,
                 "unmodifiable");
 
   // Empty list is not special.
   var l4 = [];
   Expect.throws(() { l4.removeAt(0); },
                 (e) => e is RangeError,
                 "empty");
 }