Revert "Remove Expect from core library."

tests/language/instance_compound_assignment_operator_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.
 // Test correct instance compound assignment operator.
 
-import "package:expect/expect.dart";
-
 class A {
   A() : f = 2 {}
   var f;
   operator [](index) => f;
   operator []=(index, value) => f = value;
 
   var _g = 0;
   var gGetCount = 0;
   var gSetCount = 0;
   get g {
@@ skipping 56 matching lines @@
   Expect.equals(1, b._a.gGetCount);
   Expect.equals(1, b._a.gSetCount);
   Expect.equals(1, b._a._g);
 
   globalA = b._a;
   globalA.f = 0;
   foo().f += 1;
   Expect.equals(1, fooCounter);
   Expect.equals(1, globalA.f);
 }