convert isolates to use Future

frog/lib/isolate.dart

Index: frog/lib/isolate.dart
diff --git a/frog/lib/isolate.dart b/frog/lib/isolate.dart
index 0680c740d1c4e3fd392009d8aadf8cdc4bf54dbf..ce13bf00de404592979a99b12ee2f3a0250bc026 100644
--- a/frog/lib/isolate.dart
+++ b/frog/lib/isolate.dart
@@ -153,15 +153,15 @@ class ReceivePortSingleShotImpl implements ReceivePort {
 final String _SPAWNED_SIGNAL = "spawned";
 
 class IsolateNatives {
-  static Promise<SendPort> spawn(Isolate isolate, bool isLight) {
-    Promise<SendPort> result = new Promise<SendPort>();
+  static Future<SendPort> spawn(Isolate isolate, bool isLight) {
+    Completer<SendPort> completer = new Completer<SendPort>();
     ReceivePort port = new ReceivePort.singleShot();
     port.receive((msg, SendPort replyPort) {
       assert(msg == _SPAWNED_SIGNAL);
-      result.complete(replyPort);
+      completer.complete(replyPort);
     });
     _spawn(isolate, isLight, port.toSendPort());
-    return result;
+    return completer.future;
   }
 
   static SendPort _spawn(Isolate isolate, bool light, SendPort port) native;