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

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

Index: pkg/barback/test/package_graph/transform/concurrency_test.dart
diff --git a/pkg/barback/test/package_graph/transform/concurrency_test.dart b/pkg/barback/test/package_graph/transform/concurrency_test.dart
index c659e610c2cfd8b41bd72a94a19d4c2cb0e4d062..41e3047a5c9e55d6d08b61c5890d00dd918a2927 100644
--- a/pkg/barback/test/package_graph/transform/concurrency_test.dart
+++ b/pkg/barback/test/package_graph/transform/concurrency_test.dart
@@ -470,4 +470,24 @@ main() {
     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();
+  });
 }