Work around issue 8512 in pub.

utils/pub/error_group.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.
 
 library error_group;
 
 import 'dart:async';
 
 import 'utils.dart';
 
@@ skipping 214 matching lines @@
   /// The underlying [StreamController] for [this].
   final StreamController _controller;
 
   /// The [StreamSubscription] that connects the wrapped [Stream] to
   /// [_controller].
   StreamSubscription _subscription;
 
   /// 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),
       {void onError(AsyncError error), void onDone(),
        bool unsubscribeOnError}) {
     return _controller.stream.listen(onData,
         onError: onError,
         onDone: onDone,
         unsubscribeOnError: true);
   }
 
   /// Signal that an error from [_group] should be propagated through [this],
   /// 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(() {
       _controller.signalError(e.error, e.stackTrace);
       _controller.close();
     });
   }
 }