Revert "Remove Expect from core library."

tests/language/default_implementation_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.
 // Dart test to verify that factory classes are working.
 
-import "package:expect/expect.dart";
-
 // @static-clean
 
 abstract class Point {
   factory Point(int x, int y) = PointImplementation;
 
   final int x;
   final int y;
 }
 
 class PointImplementation implements Point {
   const PointImplementation(int x, int y) : this.x = x, this.y = y;
   final int x;
   final int y;
 }
 
 class DefaultImplementationTest {
   static void testMain() {
     Point point = new Point(4, 2);
     Expect.equals(4, point.x);
     Expect.equals(2, point.y);
   }
 }
 
 main() {
   DefaultImplementationTest.testMain();
 }