Revert "Remove Expect from core library."

tests/language/arithmetic_canonicalization_test.dart

 // Copyright (c) 2013, 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 canonicalization of simple arithmetic equivalences.
 
-import "package:expect/expect.dart";
-
 main() {
   for (var i = 0; i < 5000; i++) {
     Expect.isTrue(mul1double(i) is double);
     Expect.equals(i.toDouble(), mul1double(i));
     Expect.equals(0.0, mul0double(i));
     Expect.equals(i.toDouble(), add0double(i));
 
     Expect.equals(i, mul1int(i));
     Expect.equals(i, add0int(i));
     Expect.equals(0, mul0int(i));
@@ skipping 11 matching lines @@
 mul0double(x) => 0.0 * x;
 add0double(x) => 0.0 + x;
 
 mul1int(x) => 1 * x;
 mul0int(x) => 0 * x;
 add0int(x) => 0 + x;
 and0(x) => 0 & x;
 or0(x) => 0 | x;
 xor0(x) => 0 ^ x;
 and1(x) => (-1) & x;