code: private TeaServletTemplateSource createTemplateSource(
                                           ContextSource contextSource) {
          return TeaServletTemplateSource.createTemplateSource(
              getServletContext(),
              (TeaServletContextSource) contextSource,
              getProperties().subMap("template"),
              getLog());
      }
docs: Create a template source using the composite context passed to
  the method.
  
  @param contextSource The composite context source for all the
  applications in the ApplicationDepot.
  
  @return A newly created template source.