Revert "Remove Expect from core library."

tests/language/fibo_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.
 // Dart test program calculating the Fibonacci sequence.
 
-import "package:expect/expect.dart";
-
 class Helper {
   static int fibonacci(int n) {
     int a = 0, b = 1, i = 0;
     while (i++ < n) {
       a = a + b;
       b = a - b;
     }
     return a;
   }
 }
 
 class FiboTest {
   static testMain() {
     Expect.equals(0, Helper.fibonacci(0));
     Expect.equals(1, Helper.fibonacci(1));
     Expect.equals(1, Helper.fibonacci(2));
     Expect.equals(2, Helper.fibonacci(3));
     Expect.equals(3, Helper.fibonacci(4));
     Expect.equals(5, Helper.fibonacci(5));
     Expect.equals(102334155, Helper.fibonacci(40));
   }
 }
 
 main() {
   FiboTest.testMain();
 }