Fix filtering out .dart files in release mode.

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

Index: sdk/lib/_internal/pub/lib/src/barback/server.dart
diff --git a/sdk/lib/_internal/pub/lib/src/barback/server.dart b/sdk/lib/_internal/pub/lib/src/barback/server.dart
index 7fd758f99433ce53f6094817d5e63fe47a9c06ca..f491aab1ab13291b9b78b0263cd5dd66179a891e 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/server.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/server.dart
@@ -16,6 +16,9 @@ import '../barback.dart';
 import '../log.dart' as log;
 import '../utils.dart';
 
+/// Callback for determining if an asset with [id] should be served or not.
+typedef bool AllowAsset(AssetId id);
+
 /// A server that serves assets transformed by barback.
 class BarbackServer {
   /// The underlying HTTP server.
@@ -34,6 +37,15 @@ class BarbackServer {
   /// The server's address.
   final InternetAddress address;
 
+  /// Optional callback to determine if an asset should be served.
+  ///
+  /// This can be set to allow outside code to filter out assets. Pub serve
+  /// uses this after plug-ins are loaded to avoid serving ".dart" files in
+  /// release mode.
+  ///
+  /// If this is `null`, all assets may be served.
+  AllowAsset allowAsset;
+
   /// The results of requests handled by the server.
   ///
   /// These can be used to provide visual feedback for the server's processing.
@@ -91,6 +103,12 @@ class BarbackServer {
       return;
     }
 
+    // See if the asset should be blocked.
+    if (allowAsset != null && !allowAsset(id)) {
+      _notFound(request, "Asset $id is not available in this configuration.");
+      return;
+    }
+
     _logRequest(request, "Loading $id");
     barback.getAssetById(id).then((asset) {
       return validateStream(asset.read()).then((stream) {