Add _File._deleteLink to dart2js dart:io patch.

sdk/lib/_internal/compiler/implementation/lib/io_patch.dart

Index: sdk/lib/_internal/compiler/implementation/lib/io_patch.dart
diff --git a/sdk/lib/_internal/compiler/implementation/lib/io_patch.dart b/sdk/lib/_internal/compiler/implementation/lib/io_patch.dart
index 6cd1b5005fa54d64d885d40da3f88508d00f66c9..3f0fadf6eb94ee511e0b22b0176e2e8c25a65bff 100644
--- a/sdk/lib/_internal/compiler/implementation/lib/io_patch.dart
+++ b/sdk/lib/_internal/compiler/implementation/lib/io_patch.dart
@@ -78,6 +78,9 @@ patch class _File {
   patch static _delete(String path) {
     throw new UnsupportedError("File._delete");
   }
+  patch static _deleteLink(String path) {
+    throw new UnsupportedError("File._deleteLink");
+  }
   patch static _directory(String path) {
     throw new UnsupportedError("File._directory");
   }