Remove Expect from core library.

tests/corelib/exception_implementation_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.
 
 library exception_implementation_test;
+import "package:expect/expect.dart";
 
 main() {
   final msg = 1;
   try {
     throw new Exception(msg);
     Expect.fail("Unreachable");
   } on Exception catch (e) {
     Expect.isTrue(e is Exception);
     Expect.equals("Exception: $msg", e.toString());
   }
 }