Revert "Remove Expect from core library."

tests/language/arithmetic_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.
 // Dart test program to test arithmetic operations.
 
 library arithmetic_test;
-import "package:expect/expect.dart";
 import 'dart:math';
 
 class ArithmeticTest {
 
   static bool exceptionCaughtParseInt(String s) {
     try {
       int.parse(s);
       return false;
     } on FormatException catch (e) {
       return true;
@@ skipping 423 matching lines @@
       runOne();
       testSmiDivDeopt();
       testSqrtDeopt();
     }
   }
 }
 
 main() {
   ArithmeticTest.testMain();
 }