Only run at most 10 transformers at once in barback.

pkg/barback/test/package_graph/many_parallel_transformers_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.
+
+library barback.test.package_graph.transform_test;
+
+import 'package:barback/src/utils.dart';
+import 'package:scheduled_test/scheduled_test.dart';
+
+import '../utils.dart';
+
+main() {
+  initConfig();
+
+  test("handles many parallel transformers", () {
+    var files = new List.generate(100, (i) => "app|$i.txt");
+    var rewrite = new RewriteTransformer("txt", "out");
+    initGraph(files, {"app": [[rewrite]]});
+
+    // Pause and resume apply to simulate parallel long-running transformers.
+    rewrite.pauseApply();
+    updateSources(files);
+    schedule(pumpEventQueue);
+    rewrite.resumeApply();
+
+    for (var i = 0; i < 100; i++) {
+      expectAsset("app|$i.out", "$i.out");
+    }
+    buildShouldSucceed();
+
+    expect(rewrite.maxParallelRuns, completion(equals(10)));
+  });
+}