Revert "Remove Expect from core library."

tests/language/side_effect_throw_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";
-
 class B {
   static var x;
   operator <<(other) {
     x = other;
     return 33;
   }
 }
 
 class A {
   get _m => new B();
   opshl(n) {
     // 'n' must be a number, but we are not allowed to throw before we have
     // evaluated _m << 499.
     return (_m << 499) | (2 - n);
   }
 }
 
 main() {
   var a = new A();
   Expect.throws(() => a.opshl("string"));
   Expect.equals(499, B.x);
 }