Revert "Remove Expect from core library."

tests/language/prefix14_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.
 //
 // Use qualified symbols at various places.
 
 library Prefix14Test.dart;
-import "package:expect/expect.dart";
 import "library12.dart" as lib12;
 
 typedef lib12.Library12 myFunc(lib12.Library12 param);
 
 class myInterface implements lib12.Library12Interface {
   myInterface(lib12.Library12 this.myfld);
   lib12.Library12 addObjects(lib12.Library12 value1, lib12.Library12 value2) {
     myfld.fld = (value1.fld + value2.fld + myfld.fld);
     return myfld;
   }
@@ skipping 40 matching lines @@
   Expect.equals(2, o.fld2.func());
   Expect.equals(200, o.fld2.fld);
 
   o = new myInterface(new lib12.Library12(100));
   Expect.equals(2, o.myfld.func());
   Expect.equals(100, o.myfld.fld);
   o = o.addObjects(new lib12.Library12(200), new lib12.Library12(300));
   Expect.equals(2, o.func());
   Expect.equals(600, o.fld);
 }