Work around issue 8512 in pub.

utils/pub/utils.dart

 // Copyright (c) 2012, 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.
 
 /// Generic utility functions. Stuff that should possibly be in core.
 library utils;
 
 import 'dart:async';
 import 'dart:crypto';
 import 'dart:isolate';
@@ skipping 155 matching lines @@
 /// to [completer].
 void chainToCompleter(Future future, Completer completer) {
   future.then((value) => completer.complete(value),
       onError: (e) => completer.completeError(e.error, e.stackTrace));
 }
 
 // TODO(nweiz): remove this when issue 7964 is fixed.
 /// 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;
 }
 
 /// Returns a wrapped version of [stream] along with a [StreamSubscription] that
 /// can be used to control the wrapped stream.
 Pair<Stream, StreamSubscription> streamWithSubscription(Stream stream) {
   var controller = stream.isBroadcast ?
       new StreamController.broadcast() :
       new StreamController();
   var subscription = stream.listen(controller.add,
@@ skipping 129 matching lines @@
 
 /// Add all key/value pairs from [source] to [destination], overwriting any
 /// pre-existing values.
 void mapAddAll(Map destination, Map source) =>
   source.forEach((key, value) => destination[key] = value);
 
 /// Decodes a URL-encoded string. Unlike [decodeUriComponent], this includes
 /// replacing `+` with ` `.
 String urlDecode(String encoded) =>
   decodeUriComponent(encoded.replaceAll("+", " "));