Work around issue 8512 in pub.

Review URL: https://codereview.chromium.org//12225157

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@18427 260f80e4-7a28-3924-810f-c04153c831b5

lib/src/error_group.dart

@@ -232,19 +232,27 @@ class _ErrorGroupStream extends Stream {
   /// Whether [this] has any listeners.
   bool get _hasListeners => _controller.hasSubscribers;
 
+  // TODO(nweiz): Remove this when issue 8512 is fixed.
+  /// Whether the subscription has been cancelled.
+  bool _cancelled = false;
+
   /// Creates a new [_ErrorGroupFuture] that's a child of [_group] and wraps
   /// [inner].
   _ErrorGroupStream(this._group, Stream inner)
     : _controller = inner.isBroadcast ?
           new StreamController.broadcast() :
           new StreamController() {
-    _subscription = inner.listen(_controller.add,
-        onError: (e) => _group._signalError(e),
-        onDone: () {
-          _isDone = true;
-          _group._signalStreamComplete(this);
-          _controller.close();
-        });
+    _subscription = inner.listen((v) {
+      if (!_cancelled) _controller.add(v);
+    }, onError: (e) {
+      if (!_cancelled) _group._signalError(e);
+    }, onDone: () {
+      if (!_cancelled) {
+        _isDone = true;
+        _group._signalStreamComplete(this);
+        _controller.close();
+      }
+    });
   }
 
   StreamSubscription listen(void onData(value),
@@ -260,6 +268,7 @@ class _ErrorGroupStream extends Stream {
   /// unless it's already complete.
   void _signalError(AsyncError e) {
     if (_isDone) return;
+    _cancelled = true;
     _subscription.cancel();
     // Call these asynchronously to work around issue 7913.
     defer(() {

lib/src/utils.dart

@@ -173,15 +173,25 @@ void chainToCompleter(Future future, Completer completer) {
 /// Returns a [Future] that will complete to the first element of [stream].
 /// Unlike [Stream.first], this is safe to use with single-subscription streams.
 Future streamFirst(Stream stream) {
+  // TODO(nweiz): remove this when issue 8512 is fixed.
+  var cancelled = false;
   var completer = new Completer();
   var subscription;
   subscription = stream.listen((value) {
-    subscription.cancel();
-    completer.complete(value);
-  },
-      onError: (e) => completer.completeError(e.error, e.stackTrace),
-      onDone: () => completer.completeError(new StateError("No elements")),
-      unsubscribeOnError: true);
+    if (!cancelled) {
+      cancelled = true;
+      subscription.cancel();
+      completer.complete(value);
+    }
+  }, onError: (e) {
+    if (!cancelled) {
+      completer.completeError(e.error, e.stackTrace);
+    }
+  }, onDone: () {
+    if (!cancelled) {
+      completer.completeError(new StateError("No elements"));
+    }
+  }, unsubscribeOnError: true);
   return completer.future;
 }