Revert "Remove Expect from core library."

tests/language/arithmetic2_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.
 // @static-clean
 
-import "package:expect/expect.dart";
-
 class A {
   static foo() => 499;
 }
 
 bool throwsNoSuchMethod(f) {
   try {
     f();
     return false;
   } on NoSuchMethodError catch (e) {
     return true;
@@ skipping 46 matching lines @@
   numberOpBadSecondArgument((x) => 3 - x);
   numberOpBadSecondArgument((x) => 3 * x);
   numberOpBadSecondArgument((x) => 3 / x);
   numberOpBadSecondArgument((x) => 3 ~/ x);
   numberOpBadSecondArgument((x) => 3 % x);
   badOperations(true);
   badOperations(false);
   badOperations(() => 499);
   badOperations(A.foo);
 }