Make ReceivePort an abstract class.

sdk/lib/isolate/base.dart

Index: sdk/lib/isolate/base.dart
diff --git a/sdk/lib/isolate/base.dart b/sdk/lib/isolate/base.dart
index d61c35c4a561727cd739103ef7a9763fd4f7e4bf..e43e3bbe9b0986ed4c208b6045c07cf861b58c73 100644
--- a/sdk/lib/isolate/base.dart
+++ b/sdk/lib/isolate/base.dart
@@ -105,13 +105,13 @@ abstract class SendPort {
  *
  * A [ReceivePort] may have many [SendPort]s.
  */
-interface ReceivePort default _ReceivePortFactory {
+abstract class ReceivePort {
 
   /**
    * Opens a long-lived port for receiving messages. The returned port
    * must be explicitly closed through [ReceivePort.close].
    */
-  ReceivePort();
+  external factory ReceivePort();
 
   /**
    * Sets up a callback function for receiving pending or future
@@ -142,7 +142,3 @@ abstract class SendPortSync {
   callSync(var message);
 
 }
-
-class _ReceivePortFactory {
-  external factory ReceivePort();
-}