Remove Expect from core library.

tests/language/super_assign_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 {
   int x;
 }
 
 class C extends A {
   void setX(int value) { super.x = value; }
 }
 
 main() {
  A a = new C();
  a.x = 37;
  a.setX(42);
  Expect.equals(42, a.x);
 }