Add a TODO to the code added by r36460.

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

Index: sdk/lib/_internal/pub/lib/src/barback/excluding_transformer.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/excluding_transformer.dart b/sdk/lib/_internal/pub/lib/src/barback/excluding_transformer.dart
index 9182ec6d48a3ee4efba22646dffb275a649e0619..1892f97a13d1b7b6a1550596d4835a6c6b64d5a8 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/excluding_transformer.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/excluding_transformer.dart
@@ -20,6 +20,7 @@ class ExcludingTransformer extends Transformer {
     if (includes == null && excludes == null) return inner;
 
     if (inner is LazyTransformer) {
+      // TODO(nweiz): Remove these unnecessary "as"es when issue 19046 is fixed.
       return new _LazyExcludingTransformer(
           inner as LazyTransformer, includes, excludes);
     } else if (inner is DeclaringTransformer) {