Remove Expect from core library.

tests/language/const_nested_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";
+
 // Test of compile time constant local variables in outer function levels.
 
 const N = 8;
 
 class ConstFoo {
   final x;
   const ConstFoo(this.x);
 }
 
 getClosure() {
@@ skipping 20 matching lines @@
 }
 
 
 main() {
   var f = getClosure();
   f();
 }