Remove Expect from core library.

tests/language/const_escape_frog_test.dart

+// Copyright (c) 2013, 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.
+// Test division by power of two.
+// Test that results before and after optimization are the same.
+
+import "package:expect/expect.dart";
+
 class Foo {
   const Bar<Foo> bar = const Bar/* comment here use to trigger bug 323 */();
 }
 
 class Bar<T extends Foo> {
   const Bar();
 }
 
 main() {
   Expect.equals(new Foo().bar, const Bar());
 }