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

tools/dom/src/EventStreamProvider.dart

 // Copyright (c) 2013, 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 html;
 
 /**
  * Adapter for exposing DOM events as Dart streams.
  */
 class _EventStream<T extends Event> extends Stream<T> {
@@ skipping 23 matching lines @@
   final String _eventType;
   var _onData;
   final bool _useCapture;
 
   _EventStreamSubscription(this._target, this._eventType, this._onData,
       this._useCapture) {
     _tryResume();
   }
 
   void cancel() {
-    if (_canceled) {
-      throw new StateError("Subscription has been canceled.");
-    }
+    if (_canceled) return;
 
     _unlisten();
     // Clear out the target to indicate this is complete.
     _target = null;
     _onData = null;
   }
 
   bool get _canceled => _target == null;
 
   void onData(void handleData(T event)) {
     if (_canceled) {
       throw new StateError("Subscription has been canceled.");
     }
     // Remove current event listener.
     _unlisten();
 
     _onData = handleData;
     _tryResume();
   }
 
   /// Has no effect.
   void onError(void handleError(AsyncError error)) {}
 
   /// Has no effect.
   void onDone(void handleDone()) {}
 
   void pause([Future resumeSignal]) {
-    if (_canceled) {
-      throw new StateError("Subscription has been canceled.");
-    }
+    if (_canceled) return;
     ++_pauseCount;
     _unlisten();
 
     if (resumeSignal != null) {
       resumeSignal.whenComplete(resume);
     }
   }
 
   bool get _paused => _pauseCount > 0;
 
   void resume() {
-    if (_canceled) {
-      throw new StateError("Subscription has been canceled.");
-    }
-    if (!_paused) {
-      throw new StateError("Subscription is not paused.");
-    }
+    if (_canceled || !_paused) return;
     --_pauseCount;
     _tryResume();
   }
 
   void _tryResume() {
     if (_onData != null && !_paused) {
       _target.$dom_addEventListener(_eventType, _onData, _useCapture);
     }
   }
 
@@ skipping 55 matching lines @@
   const _CustomEventStreamProvider(this._eventTypeGetter);
 
   Stream<T> forTarget(EventTarget e, {bool useCapture: false}) {
     return new _EventStream(e, _eventTypeGetter(e), useCapture);
   }
 
   String getEventType(EventTarget target) {
     return _eventTypeGetter(target);
   }
 }