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

sdk/lib/async/stream_impl.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;
 
 // States shared by single/multi stream implementations.
 
 // Completion state of the stream.
 /// Initial and default state where the stream can receive and send events.
@@ skipping 924 matching lines @@
   void _sendError(AsyncError error) {
     _onError(error);
     if (_unsubscribeOnError) _source._cancel(this);
   }
 
   void _sendDone() {
     _onDone();
   }
 
   void cancel() {
+    if (!_isSubscribed) return;
     _source._cancel(this);
   }
 
   void pause([Future resumeSignal]) {
+    if (!_isSubscribed) return;
     _source._pause(this, resumeSignal);
   }
 
   void resume() {
-    if (!isPaused) {
-      throw new StateError("Resuming unpaused subscription");
-    }
+    if (!_isSubscribed || !isPaused) return;
     _source._resume(this, false);
   }
 }
 
 // Internal helpers.
 
 // Types of the different handlers on a stream. Types used to type fields.
 typedef void _DataHandler<T>(T value);
 typedef void _ErrorHandler(AsyncError error);
 typedef void _DoneHandler();
@@ skipping 297 matching lines @@
 
   void onData(void handleAction(T value)) {}
 
   void onError(void handleError(AsyncError error)) {}
 
   void onDone(void handleDone()) {
     _handler = handleDone;
   }
 
   void pause([Future signal]) {
-    if (_isComplete) {
-      throw new StateError("Subscription has been canceled.");
-    }
+    if (_isComplete) return;
     if (_timer != null) {
       _timer.cancel();
       _timer = null;
     }
     _pauseCount++;
     if (signal != null) signal.whenComplete(resume);
   }
 
   void resume() {
-    if (_isComplete) {
-      throw new StateError("Subscription has been canceled.");
-    }
+    if (_isComplete) return;
     if (_pauseCount == 0) return;
     _pauseCount--;
     if (_pauseCount == 0) {
       _delayDone();
     }
   }
 
   bool get isPaused => _pauseCount > 0;
 
   void cancel() {
-    if (_isComplete) {
-      throw new StateError("Subscription has been canceled.");
-    }
+    if (_isComplete) return;
     if (_timer != null) {
       _timer.cancel();
       _timer = null;
     }
     _pauseCount = 0;
   }
 }
 
 class _SingleStreamMultiplexer<T> extends _MultiStreamImpl<T> {
   final Stream<T> _source;
@@ skipping 24 matching lines @@
                                      onError: this._addError,
                                      onDone: this._close);
     } else {
       // TODO(lrn): Check why this can happen.
       if (_subscription == null) return;
       _subscription.cancel();
       _subscription = null;
     }
   }
 }