Remove Expect from core library.

tests/language/issue7525_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";
+
 // Regression test for issue 7525.
 
 foo() {
   var ol = <int>[2];
   (ol as List<int>)[0];
   int x = (ol as List<int>)[0];
   return x;
 }
 
 main() {
   for (int i = 0; i < 5000; i++) {
     foo();
   }
   Expect.equals(2, foo());
 }