Revert "Remove Expect from core library."

tests/language/call_constructor_on_unresolvable_class_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.
 
 // Check that calling a constructor of a class that cannot be resolved causes
 // a runtime error.
 
-import "package:expect/expect.dart";
 import 'dart:math';
 
 never() {
   Random r = new Random();
   int r1 = r.nextInt(1000);
   int r2 = r.nextInt(1000);
   int r3 = r.nextInt(1000);
   return (r1 > r3) && (r2 > r3) && (r3 > r1 + r2);
 }
 
@@ skipping 10 matching lines @@
   new lib.A();     /// 06: runtime error
 
   var ex;                    /// 07: static type warning
   try {                      /// 07: continued
     new A();                 /// 07: continued
   } catch (e) {              /// 07: continued
     ex = e;                  /// 07: continued
   }                          /// 07: continued
   Expect.isTrue(ex != null); /// 07: continued
 }