Remove Expect from core library.

tests/language/prefix9_negative_test.dart

 // Copyright (c) 2011, 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";
+
 // Local variables can shadow type parameters and hence should result in an
 // error.
 
 class Test<T> {
   Test.named(T this.fld);
   T fld;
 }
 
 class Param {
   Param.named(int this.fld);
   int fld;
 }
 
 main() {
   Param test = new Param.named(10);
   var Param;
   var i = new Test<Param>.named(test);  // This should be an error.
   Expect.equals(10, i.fld.fld);
 }