Use a pool to restrict access to file descriptors in pub.

sdk/lib/_internal/pub/lib/src/utils.dart

Index: sdk/lib/_internal/pub/lib/src/utils.dart
diff --git a/sdk/lib/_internal/pub/lib/src/utils.dart b/sdk/lib/_internal/pub/lib/src/utils.dart
index 711b224cca4ee90d18d6d8bc4c14800a143a1c3e..df9f9d121b18fd38ae7336c4d9a8ac34c415a5c8 100644
--- a/sdk/lib/_internal/pub/lib/src/utils.dart
+++ b/sdk/lib/_internal/pub/lib/src/utils.dart
@@ -77,10 +77,68 @@ class FutureGroup<T> {
   Future<List> get future => _completer.future;
 }
 
+/// Returns a buffered stream that will emit the same values as the stream
+/// returned by [future] once [future] completes.
+///
+/// If [future] completes to an error, the return value will emit that error and
+/// then close.
+///
+/// If [broadcast] is true, a broadcast stream is returned. This assumes that
+/// the stream returned by [future] will be a broadcast stream as well.
+/// [broadcast] defaults to false.
+Stream futureStream(Future<Stream> future, {bool broadcast: false}) {
+  var subscription;
+  var controller;
+
+  future = future.catchError((e, stackTrace) {
+    // Since [controller] is synchronous, it's likely that emitting an error
+    // will cause it to be cancelled before we call close.
+    if (controller != null) controller.addError(e, stackTrace);
+    if (controller != null) controller.close();
+    controller = null;
+  });
+
+  onListen() {
+    future.then((stream) {
+      if (controller == null) return;
+      subscription = stream.listen(
+          controller.add,
+          onError: controller.addError,
+          onDone: controller.close);
+    });
+  }
+
+  onCancel() {
+    if (subscription != null) subscription.cancel();
+    subscription = null;
+    controller = null;
+  }
+
+  if (broadcast) {
+    controller = new StreamController.broadcast(
+        sync: true, onListen: onListen, onCancel: onCancel);
+  } else {
+    controller = new StreamController(
+        sync: true, onListen: onListen, onCancel: onCancel);
+  }
+  return controller.stream;
+}
+
 /// Like [new Future], but avoids around issue 11911 by using [new Future.value]
 /// under the covers.
 Future newFuture(callback()) => new Future.value().then((_) => callback());
 
+/// Returns a [StreamTransformer] that will call [onDone] when the stream
+/// completes.
+///
+/// The stream will be passed through unchanged.
+StreamTransformer onDoneTransformer(void onDone()) {
+  return new StreamTransformer.fromHandlers(handleDone: (sink) {
+    onDone();
+    sink.close();
+  });
+}
+
 // TODO(rnystrom): Move into String?
 /// Pads [source] to [length] by adding spaces at the end.
 String padRight(String source, int length) {