[ffigen] Use an asset-id example that includes a package name.

[brianquinlan]

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pkgs/jni/lib/src/third_party/generated_bindings.dart
pkgs/native_doc_dartifier/example/native_doc_dartifier_example.dart
pkgs/native_doc_dartifier/lib/native_doc_dartifier.dart
pkgs/native_doc_dartifier/lib/src/native_doc_dartifier_base.dart
pkgs/native_doc_dartifier/test/native_doc_dartifier_test.dart
pkgs/objective_c/lib/src/ns_input_stream.dart