Revert "Remove Expect from core library."

tests/standalone/io/process_kill_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.
 //
 // Process test program to test process communication.
 
 library ProcessKillTest;
-import "package:expect/expect.dart";
 import "dart:io";
 import "process_test_util.dart";
 
 testKill() {
   // Start a process that will hang waiting for input until killed.
   Process.start(getProcessTestFileName(), const ["0", "1", "0", "0"]).then((p) {
     p.exitCode.then((exitCode) {
       // Process killed from the side so exit code is not 0.
       Expect.isTrue(exitCode != 0);
       // Killing a process that is already dead returns false.
       Expect.isFalse(p.kill());
     });
     Expect.isTrue(p.kill());
   });
 }
 
 main() {
   testKill();
 }