Remove Expect from core library.

tests/language/operator5_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";
+
 class A {
   operator ==(other) => 1;
   operator <(other) => null;
   operator <=(other) => 499;
   operator >(other) => "foo";
   operator >=(other) => 42;
 }
 
 // This triggered a bug in Dart2Js: equality operator was always boolified.
 equals(a) {
@@ skipping 29 matching lines @@
 }
 
 main() {
   var a = new A();
   equals(a);
   less(a);
   lessEqual(a);
   greater(a);
   greaterEqual(a);
 }