Revert "Remove Expect from core library."

tests/language/skip_expression_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 OneArg<A> {
   OneArg<A> get foo => new OneArg<A>();
   OneArg<A> get bar {
     return new OneArg<A>();
   }
 }
 
 class TwoArgs<A,B> {
   TwoArgs<A,B> get foo => new TwoArgs<A,B>();
   TwoArgs<A,B> get bar {
     return new TwoArgs<A,B>();
   }
 
 }
 
 void main() {
   Expect.isTrue(new OneArg<String>().foo is OneArg);
   Expect.isTrue(new OneArg<String>().bar is OneArg);
   Expect.isTrue(new TwoArgs<String, int>().foo is TwoArgs);
   Expect.isTrue(new TwoArgs<String, int>().bar is TwoArgs);
 
   // TODO(karlklose): Please remove the return when dart2js can handle
   // the type tests after it.
   return;
   Expect.isTrue(new OneArg<String>().foo is OneArg<String>);
   Expect.isTrue(new OneArg<String>().bar is OneArg<String>);
   Expect.isTrue(new TwoArgs<String, int>().foo is TwoArgs<String, int>);
   Expect.isTrue(new TwoArgs<String, int>().bar is TwoArgs<String, int>);
 }