Revert "Remove Expect from core library."

tests/standalone/io/process_start_exception_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 errors during startup of the process.
 
-import "package:expect/expect.dart";
 import 'dart:async';
 import 'dart:io';
 
 testStartError() {
   Future<Process> processFuture =
       Process.start("__path_to_something_that_should_not_exist__",
                     const []);
   processFuture.then((p) => Expect.fail('got process despite start error'))
   .catchError((e) {
     Expect.isTrue(e.error is ProcessException);
@@ skipping 10 matching lines @@
   .catchError((e) {
     Expect.isTrue(e.error is ProcessException);
     Expect.equals(2, e.error.errorCode, e.error.toString());
   });
 }
 
 main() {
   testStartError();
   testRunError();
 }