Remove Expect from core library.

tests/language/loop_exchange4_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";
+
 // This program tripped dart2js.
 main() {
   var count = 0;
   for (var i = 0; i < 1; i++, count += i) {}
 
   int foo;
   for (var i = 0; i < 10; foo = i, i++) {
     if(i > 0) {
       Expect.equals(i - 1, foo);
     }
   }
 }