Fix barback test failures.

pkg/barback/test/package_graph/declaring_transformer_test.dart

Index: pkg/barback/test/package_graph/declaring_transformer_test.dart
diff --git a/pkg/barback/test/package_graph/declaring_transformer_test.dart b/pkg/barback/test/package_graph/declaring_transformer_test.dart
index 13efe74d7a096b00d082a8efbab3b2e8a7a2242e..d45cbbdb595a6f184a4ac2da5788c2f8358cfef9 100644
--- a/pkg/barback/test/package_graph/declaring_transformer_test.dart
+++ b/pkg/barback/test/package_graph/declaring_transformer_test.dart
@@ -97,6 +97,9 @@ main() {
     ]});
 
     updateSources(["app|foo.in"]);
+    // Give the transformers time to declare their assets.
+    schedule(pumpEventQueue);
+
     expectAsset("app|out.one", "app|out.one");
     buildShouldSucceed();
 
@@ -114,6 +117,9 @@ main() {
     // Start [declaring] running, because its input became available.
     declaring.pauseApply();
     updateSources(["app|foo.in"]);
+    // Give the transformers time to declare their assets.
+    schedule(pumpEventQueue);
+
     expectAsset("app|out.one", "app|out.one");
     expectAssetDoesNotComplete("app|out.three");