Remove Expect from core library.

tests/language/implicit_closure_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.
 // Dart test program for testing invocation of implicit closures.
 // VMOptions=
 // VMOptions=--use_slow_path
 
+import "package:expect/expect.dart";
+
 class First {
   First(this.i) {}
   var b;
   int foo() { return i; }
   Function foo1() {
     local() {
       return i;
     }
     return local;
   }
@@ skipping 13 matching lines @@
 
     var ib = obj.foo;
     Expect.equals(obj.i, ib());
   }
 }
 
 
 main() {
   ImplicitClosureTest.testMain();
 }