Change the calling conventions for handleUnrecoverableError.

pkg/front_end/lib/src/fasta/source/outline_builder.dart

Index: pkg/front_end/lib/src/fasta/source/outline_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/source/outline_builder.dart b/pkg/front_end/lib/src/fasta/source/outline_builder.dart
index 5be03bde2ae562c4ea6271562da9202b52871ed4..232c5b51be4d3c90b638fb96b5c6708d7c8011f7 100644
--- a/pkg/front_end/lib/src/fasta/source/outline_builder.dart
+++ b/pkg/front_end/lib/src/fasta/source/outline_builder.dart
@@ -689,7 +689,7 @@ class OutlineBuilder extends UnhandledListener {
       Token recover = skipNativeClause(token);
       if (recover != null) {
         nativeMethodName = unescapeString(token.next.value);
-        return recover;
+        return newSyntheticToken(recover);
       }
     }
     return super.handleUnrecoverableError(token, kind, arguments);