[dart] add explicit source to proposal

Change-Id: I8f46f76e0ed6b494630130d0c180b7479aac6eec
diff --git a/public/lib/proposal/dart/BUILD.gn b/public/lib/proposal/dart/BUILD.gn
index 29a34ee..3666ad9 100644
--- a/public/lib/proposal/dart/BUILD.gn
+++ b/public/lib/proposal/dart/BUILD.gn
@@ -9,6 +9,7 @@
 
   sources = [
     "proposal.dart",
+    "proposal_async.dart",
     "src/proposal_builder.dart",
     "src/proposal_builder_async.dart",
   ]