Revert "Remove Expect from core library."

tests/language/abstract_getter_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 to ensure that an abstract getter is not mistaken for a field.
 
 class Foo {
   get i;  // Abstract.
 }
 
 class Bar {
 }
 
 noMethod(e) => e is NoSuchMethodError;
 
 checkIt(f) {
   Expect.throws(() { f.i = 'hi'; }, noMethod);
   Expect.throws(() { print(f.i); }, noMethod);
   Expect.throws(() { print(f.i()); }, noMethod);
 }
 
 main() {
   checkIt(new Foo());
   checkIt(new Bar());
 }