Remove DirectoryException and LinkException from dart:io and use FileException instaed.

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 a4276797b0692ce0b015a63d38db04bd4f1dace0..ec417b1482c58428fa8b3b123c05b46fc9e29965 100644
--- a/sdk/lib/_internal/pub/lib/src/barback/server.dart
+++ b/sdk/lib/_internal/pub/lib/src/barback/server.dart
@@ -109,7 +109,7 @@ class BarbackServer {
             new BarbackServerResult._failure(request.uri, id, error));
 
         // If we couldn't read the asset, handle the error gracefully.
-        if (error is FileException) {
+        if (error is FileSystemException) {
           // Assume this means the asset was a file-backed source asset
           // and we couldn't read it, so treat it like a missing asset.
           _notFound(request, error);