Revert "Remove Expect from core library."

tests/language/deopt_hoisted_smi_check_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.
 // Test deoptimization on an optimistically hoisted smi check.
 
-import "package:expect/expect.dart";
-
 sum(a, b) {
   var sum = 0;
   for (var j = 1; j < 10; j++) {
     for (var i = a; i < b; i++) {
       sum++;
     }
   }
   return sum;
 }
 
 main() {
   for (var i = 0; i < 2000; i++) Expect.equals(9, sum(1, 2));
   Expect.equals(9, sum(1.0, 2.0));  // Passing double causes deoptimization.
 }