Remove Expect from core library.

tests/compiler/dart2js_native/native_closure_identity_frog_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";
+
 typedef void MyFunctionType();
 
 class A native "*A" {
   setClosure(MyFunctionType f) native;
   check(MyFunctionType f) native;
   invoke() native;
 }
 
 makeA() native { return new A(); }
 
@@ skipping 46 matching lines @@
 
   checkUntyped(makeA(), b.instanceMethod);
   checkTyped(makeA(), b.instanceMethod);
 
   checkUntyped(makeA(), closureStatement);
   checkTyped(makeA(), closureStatement);
 
   checkUntyped(makeA(), closureExpression);
   checkTyped(makeA(), closureExpression);
 }