void WasmBinaryWriter::writeSourceMapProlog() {
  *sourceMap << "{\"version\":3,\"sources\":[";
  for (size_t i = 0; i < wasm->debugInfoFileNames.size(); i++) {
    if (i > 0) {
      *sourceMap << ",";
    }
    // TODO respect JSON string encoding, e.g. quotes and control chars.
    *sourceMap << "\"" << wasm->debugInfoFileNames[i] << "\"";
  }
  *sourceMap << "],\"names\":[],\"mappings\":\"";
}