Revert "Remove Expect from core library."

tests/language/cond_expr_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.
 // Check that conditional expressions can contain assignment expressions.
 
-import "package:expect/expect.dart";
-
 
 var e1, e2;
 
 f(a) => a < 0 ? e1 = -1 : e2 = 1;
 
 main() {
   e1 = 0;
   e2 = 0;
   var r = f(-100);
   Expect.equals(-1, r);
   Expect.equals(-1, e1);
   Expect.equals(0, e2);
 
   e1 = 0;
   e2 = 0;
   r = f(100);
   Expect.equals(1, r);
   Expect.equals(0, e1);
   Expect.equals(1, e2);
 }