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

sdk/lib/async/stream_pipe.dart

Index: sdk/lib/async/stream_pipe.dart
diff --git a/sdk/lib/async/stream_pipe.dart b/sdk/lib/async/stream_pipe.dart
index 69210ecabd1f2259571bfe6db85f32fc8942bd19..f3a8ea119ecbfdeb6d8294034ffc0648c835f7da 100644
--- a/sdk/lib/async/stream_pipe.dart
+++ b/sdk/lib/async/stream_pipe.dart
@@ -158,25 +158,20 @@ class _ForwardingStreamSubscription<S, T>
   // 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.