Make Phase.getInput in barback play nicely with the push model.

pkg/barback/test/package_graph/transform/concurrency_test.dart

 // Copyright (c) 2014, 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.
 
 /// This library contains tests for transformer behavior that relates to actions
 /// happening concurrently or other complex asynchronous timing behavior.
 library barback.test.package_graph.transform.concurrency_test;
 
 import 'package:barback/src/utils.dart';
 import 'package:scheduled_test/scheduled_test.dart';
@@ skipping 452 matching lines @@
     removeSources(["app|foo.txt"]);
 
     // Make sure the removal is processed completely before we restart rewrite2.
     schedule(pumpEventQueue);
     rewrite1.resumeApply();
 
     buildShouldSucceed();
     expectNoAsset("app|foo.out1");
     expectNoAsset("app|foo.out2");
   });
+
+  test("a transformer in a later phase gets a slow secondary input from an "
+      "earlier phase", () {
+    var rewrite = new RewriteTransformer("in", "in");
+    initGraph({
+      "app|foo.in": "foo",
+      "app|bar.txt": "foo.in"
+    }, {"app": [
+      [rewrite],
+      [new ManyToOneTransformer("txt")]
+    ]});
+
+    rewrite.pauseApply();
+    updateSources(["app|foo.in", "app|bar.txt"]);
+    expectAssetDoesNotComplete("app|bar.out");
+
+    rewrite.resumeApply();
+    expectAsset("app|bar.out", "foo.in");
+    buildShouldSucceed();
+  });
 }