Revert "Remove Expect from core library."

tests/language/optimized_hoisting_checked_mode_assert_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";
-
 // Test checked mode assertions inside loops.
 
 int foo(x, n)  {
   double z = 0.0;
   for (var i=0; i<n; i++) {
     double z = x;
   }
   return 0;
 }
 
 main() {
   for (var i=0; i<10000; i++) foo(1.0, 10);
   Expect.equals(0, foo(1.0, 10));
   Expect.equals(0, foo(2, 0));  // Must not throw in checked mode.
 }