Make sure barback's FilePool doesn't take up *all* the available FDs.

pkg/barback/lib/src/utils.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 barback.utils;
 
 import 'dart:async';
+import 'dart:io';
 
 /// A pair of values.
 class Pair<E, F> {
   E first;
   F last;
 
   Pair(this.first, this.last);
 
   String toString() => '($first, $last)';
 
@@ skipping 170 matching lines @@
     stream.listen(
         controller.add,
         onError: controller.addError,
         onDone: controller.close);
   }).catchError((e, stackTrace) {
     controller.addError(e, stackTrace);
     controller.close();
   });
   return controller.stream;
 }
+
+// TODO(nweiz): Use a built-in function when issue 14244 is fixed.
+int get maxFileDescriptors {
+  if (_maxFileDescriptors != null) return _maxFileDescriptors;
+
+  // Running "sh -c ulimit -n" via the command line always reports "unlimited",
+  // even when that's clearly false. Instead we fall back on OS-based
+  // heuristics.
+  if (Platform.isWindows) {
+    _maxFileDescriptors = 512;
+    return _maxFileDescriptors;
+  } else if (Platform.isMacOS) {
+    _maxFileDescriptors = 16348;
+    return _maxFileDescriptors;
+  } else {
+    _maxFileDescriptors = 256;
+    return _maxFileDescriptors;
+  }
+}
+int _maxFileDescriptors;