syscache.js

;(function(){
  function hash(str) {
    // Source: http://stackoverflow.com/a/7616484/502126
    var hash = 0, i, chr, len
    if (str.length === 0) return hash
    for (i = 0, len = str.length; i < len; i++) {
      chr   = str.charCodeAt(i)
      hash  = ((hash << 5) - hash) + chr
      hash |= 0
    }
    return hash.toString()
  }

  function storeCache(hashCode, translated, format) {
    localStorage['syscache-' + hashCode + '-code'] = translated
    localStorage['syscache-' + hashCode + '-format'] = format || 'undefined'
  }

  function loadCache(hashCode) {
    var format = localStorage['syscache-' + hashCode + '-format']
    if (format) {
      return {
        translated: localStorage['syscache-' + hashCode + '-code'],
        format: format === 'undefined' ? undefined: format
      }
    }
    return false
  }

  System.originalTranslate = System.translate;
  System.translate = function (load) {
    var hashCode = hash(load.source)
    var cache = loadCache(hashCode)
    if (cache) {
      load.metadata.format = cache.format
      return cache.translated
    }
    return System.originalTranslate.apply(this, [load]).then(function(translated) {
      storeCache(hashCode, translated, load.metadata.format)
      console.log('Cached', load.address)
      return translated
    })
  }
})()

Love this idea. I created an IndexedDB variant here.