Revert "Remove Expect from core library."

tests/language/const_counter_negative_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.
 // Bug: 4254106 Constant constructors must have (implicit) const parameters.
 
-import "package:expect/expect.dart";
-
 class ConstCounter {
   // Incorrect assignment of a non const function to a final field.
   const ConstCounter(int i) : nextValue_ = (() => i++);
 
   final nextValue_;
 
   int nextValue() { return nextValue_(); }
 }
 
 class ConstCounterNegativeTest {
   static testMain() {
     ConstCounter cc = const ConstCounter(3);
     Expect.equals(3, cc.nextValue());
   }
 }
 
 main() {
   ConstCounterNegativeTest.testMain();
 }