Work around issue 8512 in pub.

utils/pub/error_group.dart

Index: utils/pub/error_group.dart
diff --git a/utils/pub/error_group.dart b/utils/pub/error_group.dart
index 90edfa5179678e276142788b61472519e1f06280..16b96b19c36dd3c1ddcb70c54e7b535e1db6c865 100644
--- a/utils/pub/error_group.dart
+++ b/utils/pub/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(() {