Add explicit return value.

sdk/lib/_internal/pub/lib/src/command/build.dart

Index: sdk/lib/_internal/pub/lib/src/command/build.dart
diff --git a/sdk/lib/_internal/pub/lib/src/command/build.dart b/sdk/lib/_internal/pub/lib/src/command/build.dart
index 4b76909f9dae02de7ac68cd3b68556611a445909..7a6685a60a69ec730d8f198baadd402b9b2ee40d 100644
--- a/sdk/lib/_internal/pub/lib/src/command/build.dart
+++ b/sdk/lib/_internal/pub/lib/src/command/build.dart
@@ -128,7 +128,9 @@ class BuildCommand extends BarbackCommand {
   Future _writeAsset(Asset asset) async {
     // In release mode, strip out .dart files since all relevant ones have been
     // compiled to JavaScript already.
-    if (mode == BarbackMode.RELEASE && asset.id.extension == ".dart") return;
+    if (mode == BarbackMode.RELEASE && asset.id.extension == ".dart") {
+      return null;
+    }
 
     var destPath = _idToPath(asset.id);