Fix filtering out .dart files in release mode.

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

Index: sdk/lib/_internal/pub/lib/src/command/serve.dart
diff --git a/sdk/lib/_internal/pub/lib/src/command/serve.dart b/sdk/lib/_internal/pub/lib/src/command/serve.dart
index 11a2e2aaf804b44ce07341b9fad749960ec1518c..f6df4ce2fe875ac7821f58fbd8065b3c0286a705 100644
--- a/sdk/lib/_internal/pub/lib/src/command/serve.dart
+++ b/sdk/lib/_internal/pub/lib/src/command/serve.dart
@@ -80,6 +80,12 @@ class ServeCommand extends PubCommand {
           builtInTransformers: builtInTransformers,
           watcher: watcherType);
     }).then((server) {
+      // In release mode, strip out .dart files since all relevant ones have
+      // been compiled to JavaScript already.
+      if (mode == BarbackMode.RELEASE) {
+        server.allowAsset = (url) => !url.path.endsWith(".dart");
+      }
+
       /// This completer is used to keep pub running (by not completing) and
       /// to pipe fatal errors to pub's top-level error-handling machinery.
       var completer = new Completer();