Remove Expect from core library.

tests/language/import_core_prefix_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.
 // Test explicit import of dart:core in the source code..
 
 library ImportCorePrefixTest.dart;
+import "package:expect/expect.dart";
 import "dart:core" as mycore;
 
 class Object {
 }
 
 class Map {
   Map(this._lat, this._long);
 
   get isPrimeMeridian => _long == 0;
 
   var _lat;
   var _long;
 }
 
 
 void main() {
   var test = new mycore.Map<mycore.int,mycore.String>();
   mycore.bool boolval = false;
   mycore.int variable = 10;
   mycore.num value = 10;
   mycore.dynamic d = null;
   mycore.print(new mycore.Object());
   mycore.print(new Object());
 
   var greenwich = new Map(51, 0);
   var kpao = new Map(37, -122);
-  mycore.Expect.isTrue(greenwich.isPrimeMeridian);
-  mycore.Expect.isFalse(kpao.isPrimeMeridian);
+  Expect.isTrue(greenwich.isPrimeMeridian);
+  Expect.isFalse(kpao.isPrimeMeridian);
 }