Revert "Remove Expect from core library."

tests/standalone/io/process_run_output_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.
 //
 // Test script for testing that output is handled correctly for
 // non-interactive processes started with Process.run.
 
-import "package:expect/expect.dart";
 import "dart:io";
 import "process_test_util.dart";
 
 checkOutput(encoding, output) {
   if (encoding == 'ascii') {
     Expect.equals(output, 'abc');
   } else if (encoding == 'latin1') {
     Expect.equals(output, 'æøå');
   } else if (encoding == 'utf8') {
     Expect.listEquals(output.codeUnits, [955]);
@@ skipping 40 matching lines @@
   }
   Expect.isTrue(scriptFile.existsSync());
   test(scriptFile.path, 'ascii', 'stdout');
   test(scriptFile.path, 'ascii', 'stderr');
   test(scriptFile.path, 'latin1', 'stdout');
   test(scriptFile.path, 'latin1', 'stderr');
   test(scriptFile.path, 'utf8', 'stdout');
   test(scriptFile.path, 'utf8', 'stderr');
 
 }