Only run [declareOutputs] once for each asset/transformer pair.

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

Index: pkg/barback/test/package_graph/transform/pass_through_test.dart
diff --git a/pkg/barback/test/package_graph/transform/pass_through_test.dart b/pkg/barback/test/package_graph/transform/pass_through_test.dart
index eb90418385fd1d0b89e218f5253f261f46f2840a..1e5f4b39fa7ff4c12a5d333d85f9263f4efcacb6 100644
--- a/pkg/barback/test/package_graph/transform/pass_through_test.dart
+++ b/pkg/barback/test/package_graph/transform/pass_through_test.dart
@@ -252,22 +252,4 @@ 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();
-  });
 }