Remove Expect from core library.

tests/language/compile_time_constant_h_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";
+
 class A implements B {
   final x;
   const A(this.x);
 }
 
 abstract class B {
   const factory B(x) = A;
 }
 
 const b1 = const B(499);
 
 main() {
   Expect.equals(499, b1.x);
 }