Revert "Remove Expect from core library."

tests/language/compound_assignment_operator_test.dart

 // Copyright (c) 2011, 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.
 // Tests that lhs of a compound assignement is executed only once.
 
-import "package:expect/expect.dart";
-
 
 class Indexed {
   Indexed() : _f = new List(10), count = 0 {
     _f[0] = 100;
     _f[1] = 200;
   }
   operator [](i) {
     count++;
     return _f;
   }
@@ skipping 63 matching lines @@
       testIndexed();
       testIndexedMore();
       testIndexedMoreMore();
     }
   }
 }
 
 main() {
   CompoundAssignmentOperatorTest.testMain();
 }