Fix an analysis error in pub.

sdk/lib/_internal/pub/lib/src/barback/asset_environment.dart

Index: sdk/lib/_internal/pub/lib/src/barback/asset_environment.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/asset_environment.dart b/sdk/lib/_internal/pub/lib/src/barback/asset_environment.dart
index 035dbe6dda4f89d488db192e5b4c2b2d04ba3c88..c07d8600244d3ae048ae7aa933d5e6ceea805b75 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/asset_environment.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/asset_environment.dart
@@ -266,7 +266,7 @@ class AssetEnvironment {
     }
 
     log.fine("Executables for $packageName: $executableIds");
-    if (executableIds.isEmpty) return [];
+    if (executableIds.isEmpty) return {};
 
     var package = graph.packages[packageName];
     var server = await servePackageBinDirectory(packageName);