Loads a single static asset from either a filename or URL

@param {string} source The source document to load
@return {Promise.<string>} The file contents
function loadSingle(source) {
  var promise;

  if (source.match(/^https?:\/\//)) {
    promise = httpGetPromise(source);
  } else {
    promise = readFilePromise(normalizePath(source));
  }

  return promise;
}