Make Directory.list pull-based, making it possible to pause, resume and cancel directory listing.

tests/standalone/io/directory_list_pause_test.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.
+
+import "package:expect/expect.dart";
+import "dart:async";
+import "dart:io";
+import "dart:isolate";
+
+
+void testPauseList() {
+  var keepAlive = new ReceivePort();
+  new Directory("").createTemp().then((d) {
+    int total = 8 * 1024 + 1;

[Søren Gjesse, 2013/06/17 06:37:47]

Why so many files for this test? In principle 128 + 1 should be enough.

[Anders Johnsen, 2013/06/17 07:27:11]

My computer reads 2K by default. Changed to 2K + 1.

+    for (int i = 0; i < total; i++) {
+      new File("${d.path}/$i").createSync();
+    }
+    bool first = true;
+    var subscription;
+    int count = 0;
+    subscription = d.list().listen((file) {
+      if (first) {
+        first = false;
+        subscription.pause();
+        Timer.run(() {
+          for (int i = 0; i < total; i++) {
+            new File("${d.path}/$i").deleteSync();
+          }
+          subscription.resume();
+        });
+      }
+      count++;
+    }, onDone: () {
+      Expect.isTrue(count < total);
+      keepAlive.close();
+      d.delete().then((ignore) => keepAlive.close());
+    });
+  });
+}
+
+void main() {
+  testPauseList();
+}