Revert "Remove Expect from core library."

tests/language/prefix15_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 with generics at various places.
 
 library Prefix15Test.dart;
-import "package:expect/expect.dart";
 import "library12.dart" as lib12;
 
 typedef T myFunc<T>(T param);
 
 class myInterface<T> implements lib12.Library12Interface {
   myInterface(T this.myfld);
   T addObjects(T value1, T value2) {
     myfld.fld = (value1.fld + value2.fld + myfld.fld);
     return myfld;
   }
@@ skipping 16 matching lines @@
   Expect.equals(2, o.fld2.func());
   Expect.equals(200, o.fld2.fld);
 
   o = new myInterface<lib12.Library12>(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);
 }