Revert "Remove Expect from core library."

tests/standalone/io/process_exit_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 ProcessExitTest;
-import "package:expect/expect.dart";
 import "dart:io";
 import "process_test_util.dart";
 
 testExit() {
   var future = Process.start(getProcessTestFileName(),
                              const ["0", "0", "99", "0"]);
   future.then((process) {
     process.exitCode.then((int exitCode) {
       Expect.equals(exitCode, 99);
     });
     process.stdout.listen((_) {});
     process.stderr.listen((_) {});
   });
 }
 
 testExitRun() {
   Process.run(getProcessTestFileName(),
               const ["0", "0", "99", "0"]).then((result) {
     Expect.equals(result.exitCode, 99);
     Expect.equals(result.stdout, '');
     Expect.equals(result.stderr, '');
   });
 }
 
 main() {
   testExit();
   testExitRun();
 }