Revert "Remove Expect from core library."

tests/compiler/dart2js_native/native_library_same_name_used_frog_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.
 
 // Test for correct hidden native class when interface has same name.
 
 library main;
-import "package:expect/expect.dart";
 import 'native_library_same_name_used_lib1.dart';
 
 void setup() native """
   // This code is all inside 'setup' and so not accesible from the global scope.
   function I(){}
   I.prototype.read = function() { return this._x; };
   I.prototype.write = function(x) { this._x = x; };
   makeI = function(){return new I};
 """;
 
@@ skipping 16 matching lines @@
 
   Expect.isFalse(ob is I,      'ob is I');
   Expect.isFalse(ob is ProxyI, 'ob is ProxyI');
 
   Expect.isTrue(b1 is I,       'b1 is I');
   Expect.isTrue(b1 is ProxyI,  'b1 is ProxyI');
 
   Expect.isTrue(a1 is I,       'a1 is I');
   Expect.isFalse(a1 is ProxyI, 'a1 is ProxyI');
 }