Remove Expect from core library.

tests/compiler/dart2js_extra/closure4_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";
+
 closure0() {
   var f = fib(i) {
     if (i < 2) return i;
     return fib(i - 1) + fib(i - 2);
   };
   Expect.equals(5, f(5));
 }
 
 closure1() {
   var decr1 = 0;
@@ skipping 17 matching lines @@
   };
   var g = f(true);
   Expect.equals(499, g());
 }
 
 main() {
   closure0();
   closure1();
   closure2();
 }