Revert "Remove Expect from core library."

tests/standalone/io/process_stderr_test.dart

 // Copyright (c) 2013, 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.
 //
 // VMOptions=
 // VMOptions=--short_socket_read
 // VMOptions=--short_socket_write
 // VMOptions=--short_socket_read --short_socket_write
 
-import "package:expect/expect.dart";
 import 'dart:async';
 import 'dart:io';
 import 'dart:math';
 
 import "process_test_util.dart";
 
 void test(Future<Process> future, int expectedExitCode) {
   future.then((process) {
     process.exitCode.then((exitCode) {
       Expect.equals(expectedExitCode, exitCode);
@@ skipping 37 matching lines @@
 
   // Run the test using the dart binary with an echo script.
   // The test runner can be run from either the root or from runtime.
   var scriptFile = new File("tests/standalone/io/process_std_io_script.dart");
   if (!scriptFile.existsSync()) {
     scriptFile = new File("../tests/standalone/io/process_std_io_script.dart");
   }
   Expect.isTrue(scriptFile.existsSync());
   test(Process.start(new Options().executable, [scriptFile.path, "1"]), 0);
 }