Don't wait unnecessarily.

corelib/src/implementation/promise_implementation.dart

 // Copyright (c) 2011, 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.
 
 // Dart core library.
 
 class PromiseImpl<T> implements Promise<T> {
 
   // Enumeration of possible states:
   static final int CREATED = 0;
@@ skipping 244 matching lines @@
     addCompleteHandler((val) {
       promises.forEach((promise) {
         if (!promise.isDone()) {
           promise.cancel();
         }
       });
     });
   }
 }
 
-class ProxyBase {
+// For now, extend Promise<bool> rather than either
+// a) create a new base, Completable, for Promise and Proxy, or
+// b) extend Promise<SendPort> which would expose the port.
+class ProxyBase extends PromiseImpl<bool> {
 
   ProxyBase.forPort(SendPort port) {
     _promise = new Promise<SendPort>();
     _promise.complete(port);
+    complete(true);
   }
 
   // Construct a proxy for a message reply; see the [Proxy.forReply]
   // documentation for more details.
   ProxyBase.forReply(Promise<SendPort> port) {
     _promise = port;
+    port.addCompleteHandler((_) => complete(true));
   }
 
-  // Note that comparing proxies or using them in maps is illegal
-  // until they complete.
+  // Note that comparing proxies or using them in maps or sets is
+  // illegal until they complete.
   bool operator ==(var other) {
     return (other is ProxyBase) && _promise.value == other._promise.value;
   }
 
   int hashCode() => _promise.value.hashCode();
 
-  // TODO: consider making this extend Promise<SendPort> instead?
-  void addCompleteHandler(void completeHandler()) {
-    _promise.addCompleteHandler((_) => completeHandler());
-  }
-
   static ReceivePort register(Dispatcher dispatcher) {
     if (_dispatchers === null) {
       _dispatchers = new Map<SendPort, Dispatcher>();
     }
     ReceivePort result = new ReceivePort();
     _dispatchers[result.toSendPort()] = dispatcher;
     return result;
   }
 
   get local() {
@@ skipping 69 matching lines @@
         } 
       }
       return process(marshalled);
     }).flatten();
   }
 
   Promise<SendPort> _promise;
   static Map<SendPort, Dispatcher> _dispatchers;
 
 }