Remove Expect from core library.

tests/language/parameter_initializer6_negative_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";
+
 // It is a compile-time error if a named formal parameter begins with an '_'
 
 class Foo {
   num _y;
   Foo.optional_private({this._y: 77}) {}
 }
 
 main() {
   var obj;
   obj = new Foo.optional_private(_y: 222);
   Expect.equals(222, obj._y);
 
   obj = new Foo.optional_private();
   Expect.equals(77, obj._y);
 }