Revert "Remove Expect from core library."

tests/standalone/io/process_check_arguments_script.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.
 //
 // Utility script to check that arguments are correctly passed from
 // one dart process to another using the dart:io process interface.
 import "dart:math";
 
-class Expect {
-  static void isTrue(x) {
-    if (!x) {
-      throw new Error("Not true");
-    }
-  }
-
-  static void equals(x, y) {
-    if (x != y) {
-      throw new Error("Not equal");
-    }
-  }
-}
-
 main() {
   var options = new Options();
   Expect.isTrue(options.script.endsWith('process_check_arguments_script.dart'));
   var expected_num_args = int.parse(options.arguments[0]);
   var contains_quote = int.parse(options.arguments[1]);
   Expect.equals(expected_num_args, options.arguments.length);
   for (var i = 2; i < options.arguments.length; i++) {
     Expect.isTrue((contains_quote == 0) || options.arguments[i].contains('"'));
   }
 }