Add Capability class.

sdk/lib/isolate/capability.dart

Index: sdk/lib/isolate/capability.dart
diff --git a/sdk/lib/isolate/capability.dart b/sdk/lib/isolate/capability.dart
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+++ b/sdk/lib/isolate/capability.dart
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+// Copyright (c) 2014, 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.
+
+part of dart.isolate;
+
+/**
+ * An unforgeable object that comes back as equal when passed through other
+ * isolates.
+ *
+ * Sending a capability object to another isolate, and getting it back,
+ * will produce an object that is equal to the original.
+ * There is no other way to create objects equal to a capability object.
+ *
+ * Capabilities can be used as access guards: A remote isolate can send
+ * a request for an operation, but it is only allowed if the request contains
+ * the correct capability object.
+ *
+ * This allows exposing the same interface to multiple clients,
+ * but restricting some operations to only those clients
+ * that have also been given the corresponding capability.
+ *
+ * Capabilities can be used inside a single isolate,
+ * but they have no advantage over
+ * just using `new Object` to create a unique object,
+ * and it offers no real security against other code
+ * running in the same isolate.
+ */
+class Capability {
+  /**
+   * Create a new unforgeable capability object.
+   */
+  external factory Capability();
+}