Revert "Remove Expect from core library."

tests/language/const_init_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.
 // Check that initializers of static const fields are compile time constants.
 
-import "package:expect/expect.dart";
-
 class Point {
   final x_;
   final y_;
   const Point(x, y) : x_ = x, y_ = y;
 }
 
 class ConstInitTest {
   static const N = 1;
   static const O = 1 + 3;
   static const P = 2 * (O - N);
   static const Q = const Point(0, 0);
 
   static const Q2 = const Point(0, 0);
   static const P2 = 2 * (O - N);
   static const O2 = 1 + 3;
   static const N2 = 1;
 
   static testMain() {
     Expect.equals(1, N);
     Expect.equals(4, O);
     Expect.equals(6, P);
     Expect.equals(0, Q.x_);
     Expect.equals(0, Q.y_);
   }
 }
 
 main() {
   ConstInitTest.testMain();
 }