diff --git a/sdk/lib/isolate/isolate.dart b/sdk/lib/isolate/isolate.dart
index 3ae06e9b7c7..ea6a4df773d 100644
--- a/sdk/lib/isolate/isolate.dart
+++ b/sdk/lib/isolate/isolate.dart
@@ -272,9 +272,10 @@ class Isolate {
  * a [SendPort] is delivered to its corresponding [ReceivePort]. There might be
  * many [SendPort]s for the same [ReceivePort].
  *
- * [SendPort]s can be transmitted to other isolates.
+ * [SendPort]s can be transmitted to other isolates, and they preserve equality
+ * when sent.
  */
-abstract class SendPort {
+abstract class SendPort implements Capability {
 
   /**
    * Sends an asynchronous [message] through this send port, to its
diff --git a/tests/isolate/capability_test.dart b/tests/isolate/capability_test.dart
index a9d3ae63ec8..78a8caebe1f 100644
--- a/tests/isolate/capability_test.dart
+++ b/tests/isolate/capability_test.dart
@@ -8,15 +8,25 @@ import "package:async_helper/async_helper.dart";
 
 void main() {
   asyncStart();
-  var c1 = new Capability();
-  var c2 = new Capability();
-  Expect.notEquals(c1, c2);
 
-  var receive = new RawReceivePort();
-  receive.sendPort.send(c1);
-  receive.handler = (c3) {
-    Expect.equals(c3, c1);
-    Expect.notEquals(c3, c2);
-    asyncEnd();
-  };
+  test(c1, c2) {
+    asyncStart();
+    Expect.notEquals(c1, c2);
+    var receive = new RawReceivePort();
+    receive.sendPort.send(c1);
+    receive.handler = (c3) {
+      Expect.equals(c3, c1);
+      Expect.notEquals(c3, c2);
+      receive.close();
+      asyncEnd();
+    };
+  }
+  Capability c1 = new Capability();
+  Capability c2 = new Capability();
+  Capability c3 = (new RawReceivePort()..close()).sendPort;
+  Capability c4 = (new RawReceivePort()..close()).sendPort;
+  test(c1, c2);
+  test(c3, c4);
+  test(c1, c3);
+  asyncEnd();
 }