function processOrCache(sourceText, moduleId, options, opt_url) {
  if (ENABLE_CACHING) {
    var sourceHash = stringHash(sourceText);
    var resultText = loadFromCache(sourceHash);
    if (resultText) {
      return fixupModuleIds(moduleId, resultText);
    }
  }

  resultText = global['wtfi']['process'](
      sourceText, options, opt_url);
  if (ENABLE_CACHING) {
    saveToCache(sourceHash, resultText);
  }
  return fixupModuleIds(moduleId, resultText);
}
Processes source code for instrumentation, with caching if possible.
@param {string} sourceText Input source text.
@param {number} moduleId Unique module ID.
@param {!Object} options Instrumentation options.
@param {string=} opt_url Source URL.
@return {string} Transformed source text.