Revert "Remove Expect from core library."

tests/standalone/io/test_extension_tester.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.
 
 library test_extension_test;
 
 import "test_extension.dart";
 
-class Expect {
-  static void equals(expected, actual, [msg]) {
-    if (expected != actual) {
-      if (msg == null) msg = "Expected: $expected. Actual: $actual";
-      throw new RuntimeError(msg);
-    }
-  }
-
-  static void isNull(x, [msg]) {
-    if (x != null) {
-      if (msg != null) msg = "$x not null";
-      throw new RuntimeError(msg);
-    }
-  }
-}
-
 main() {
   Expect.equals('cat 13', new Cat(13).toString(), 'new Cat(13).toString()');
 
   Expect.equals(3, Cat.ifNull(null, 3), 'Cat.ifNull(null, 3)');
   Expect.equals(4, Cat.ifNull(4, null), 'Cat.ifNull(4, null)');
   Expect.equals(5, Cat.ifNull(5, 9), 'Cat.ifNull(5, 9)');
   Expect.isNull(Cat.ifNull(null, null), 'Cat.ifNull(null, null)');
 
   try {
     Cat.throwMeTheBall("ball");
   } on String catch (e) {
     Expect.equals("ball", e);
   }
 }