Revert "Remove Expect from core library."

tests/language/compile_time_constant10_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.
 
-import "package:expect/expect.dart";
-
 // Test that 'identical(a,b)' is a compile-time constant.
 
 class C {
   final x;
   const C(this.x);
   static f3(){}
   static f4(){}
 }
 
 const i1 = 1;
@@ skipping 83 matching lines @@
     trueTests[i].test(Expect.isTrue, "true[$i]");
   }
   for (int i = 0; i < falseTests.length; i++) {
     falseTests[i].test(Expect.isFalse, "false[$i]");
   }
 
   var x = idtest;  /// 01: continued
   var x = T.idtest2;  /// 02: continued
 }