Revert 'Port the remaining of dart:core to the unified corelib.'

corelib/unified/core/implementation/expando.dart

Index: corelib/unified/core/implementation/expando.dart
diff --git a/corelib/unified/core/implementation/expando.dart b/corelib/unified/core/implementation/expando.dart
deleted file mode 100644
index bea086c30971b728f58df3f0b5e3ecb5e30bce90..0000000000000000000000000000000000000000
--- a/corelib/unified/core/implementation/expando.dart
+++ /dev/null
@@ -1,13 +0,0 @@
-// 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.
-
-class ExpandoImplementation<T> implements Expando<T> {
-  final String name;
-
-  const ExpandoImplementation([this.name]);
-
-  external T operator[](Object object);
-  external void operator[]=(Object object, T value);
-  external String toString();
-}