Add explicit return value.

R=sigurdm@google.com

Review URL: https://codereview.chromium.org//942773003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@43904 260f80e4-7a28-3924-810f-c04153c831b5

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);