code: public static Builder builder() {
      return new AutoValue_Source.Builder()
          .setPath(DEV_NULL)
          .setCode("")
          .setOriginalCodeSupplier(null)
          .setSourceMap("")
          .setSourceUrl("")
          .setSourceMappingUrl("")
          .setRuntimes(ImmutableSet.of())
          .setLoadFlags(ImmutableMap.of())
          .setEstimatedSize(0);
    }
docs: Makes a new empty builder.