code: VmSpec vmSpec() {
       ImmutableMap<String, String> options = vmOptionsBuilder.build();
      checkState(!options.isEmpty());
      return new VmSpec.Builder()
          .addAllProperties(vmProperties.get())
          .addAllOptions(options)
          .build();
    }
docs: Returns a {@link VmSpec} based on the data gathered by this visitor.
  
  @throws IllegalStateException if not all the data has been gathered.