Expose the WeakProperty type.

runtime/lib/weak_property.dart

Index: runtime/lib/weak_property.dart
diff --git a/runtime/lib/weak_property.dart b/runtime/lib/weak_property.dart
new file mode 100644
index 0000000000000000000000000000000000000000..b3e4bfd23c3605a47d66dbb57ddc3d2d6ef9ffcf
--- /dev/null
+++ b/runtime/lib/weak_property.dart
@@ -0,0 +1,15 @@
+// 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 WeakProperty {
+  factory WeakProperty(key, value) => _new(key, value);
+
+  get key() => _getKey();
+  get value() => _getValue();
+
+  static WeakProperty _new(key, value) native "WeakProperty_new";
+
+  _getKey() native "WeakProperty_getKey";
+  _getValue() native "WeakProperty_getValue";
+}