Make stream subscription more lax wrt. calling methods on it after it's completed.

sdk/lib/async/stream_pipe.dart

 // Copyright (c) 2012, 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.async;
 
 /** Utility function to create an [AsyncError] if [error] isn't one already. */
 AsyncError _asyncError(Object error, Object stackTrace, [AsyncError cause]) {
   if (error is AsyncError) return error;
   if (cause == null) return new AsyncError(error, stackTrace);
@@ skipping 140 matching lines @@
     // Don't unsubscribe on incoming error, only if we send an error forwards.
     _subscription =
         _stream._source.listen(_handleData,
                                onError: _handleError,
                                onDone: _handleDone);
   }
 
   // StreamSubscription interface.
 
   void pause([Future resumeSignal]) {
-    if (_subscription == null) {
-      throw new StateError("Subscription has been unsubscribed");
-    }
+    if (_subscription == null) return;
     _subscription.pause(resumeSignal);
   }
 
   void resume() {
-    if (_subscription == null) {
-      throw new StateError("Subscription has been unsubscribed");
-    }
+    if (_subscription == null) return;
     _subscription.resume();
   }
 
   void cancel() {
-    if (_subscription == null) {
-      throw new StateError("Subscription has been unsubscribed");
+    if (_subscription != null) {
+      _subscription.cancel();
+      _subscription = null;
     }
-    _subscription.cancel();
-    _subscription = null;
   }
 
   // _EventOutputSink interface. Sends data to this subscription.
 
   void _sendData(T data) {
     _onData(data);
   }
 
   void _sendError(AsyncError error) {
     _onError(error);
@@ skipping 328 matching lines @@
 
   void handleError(AsyncError error, EventSink<T> sink) {
     _handleError(error, sink);
   }
 
   void handleDone(EventSink<T> sink) {
     _handleDone(sink);
   }
 }