Run pub tests against older versions of barback.

third_party/pkg/barback-0.13.0/test/package_graph/transform/pass_through_test.dart

Index: third_party/pkg/barback-0.13.0/test/package_graph/transform/pass_through_test.dart
diff --git a/pkg/barback/test/package_graph/transform/pass_through_test.dart b/third_party/pkg/barback-0.13.0/test/package_graph/transform/pass_through_test.dart
similarity index 93%
copy from pkg/barback/test/package_graph/transform/pass_through_test.dart
copy to third_party/pkg/barback-0.13.0/test/package_graph/transform/pass_through_test.dart
index 1e5f4b39fa7ff4c12a5d333d85f9263f4efcacb6..eb90418385fd1d0b89e218f5253f261f46f2840a 100644
--- a/pkg/barback/test/package_graph/transform/pass_through_test.dart
+++ b/third_party/pkg/barback-0.13.0/test/package_graph/transform/pass_through_test.dart
@@ -252,4 +252,22 @@ main() {
     expectAsset("app|foo.txt", "bar.in");
     buildShouldSucceed();
   });
+
+  test("doesn't return an overwritten asset until we know it will still "
+      "be overwritten", () {
+    var rewrite = new RewriteTransformer("txt", "txt");
+    initGraph(["app|foo.txt"], {"app": [[rewrite]]});
+
+    updateSources(["app|foo.txt"]);
+    expectAsset("app|foo.txt", "foo.txt");
+    buildShouldSucceed();
+
+    rewrite.pauseIsPrimary("app|foo.txt");
+    updateSources(["app|foo.txt"]);
+    expectAssetDoesNotComplete("app|foo.txt");
+
+    rewrite.resumeIsPrimary("app|foo.txt");
+    expectAsset("app|foo.txt", "foo.txt");
+    buildShouldSucceed();
+  });
 }