Increase barback scheduled_test timeout for many_parallel_transformers test.

pkg/barback/test/package_graph/many_parallel_transformers_test.dart

Index: pkg/barback/test/package_graph/many_parallel_transformers_test.dart
diff --git a/pkg/barback/test/package_graph/many_parallel_transformers_test.dart b/pkg/barback/test/package_graph/many_parallel_transformers_test.dart
index 389beb15b44bcdbdad9d1b65386e6b7583753b22..72f19ac95a8028e6f135690347aa31737d0d6b0b 100644
--- a/pkg/barback/test/package_graph/many_parallel_transformers_test.dart
+++ b/pkg/barback/test/package_graph/many_parallel_transformers_test.dart
@@ -13,6 +13,7 @@ main() {
   initConfig();
 
   test("handles many parallel transformers", () {
+    currentSchedule.timeout *= 3;
     var files = new List.generate(100, (i) => "app|$i.txt");
     var rewrite = new RewriteTransformer("txt", "out");
     initGraph(files, {"app": [[rewrite]]});