Revert "Remove Expect from core library."

tests/language/operator3_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.
 
-import "package:expect/expect.dart";
-
 class A {
   operator -() => this;
   toString() => "5";
   abs() => "correct";
 }
 
 // This triggered a bug in Dart2Js: the speculative type optimization assigned
 // type "number" to 'a' and then to '-a'. In the bailout version the type for
 // 'a' was removed, but the type for '-a' was kept.
 foo(a) => -(-a);
 
 main() {
   Expect.equals("correct", foo(new A()).abs());
 }