message: add missing deps marked and prism
diff: "npm": "@polymer/iron-demo-helpers",
  "semver": "^3.0.0-pre.8"
  },
  + "marked": {
  + "npm": "marked",
  + "semver": "~0.3.9"
  + },
  "marked-element": {
  "npm": "@polymer/marked-element",
  "semver": "^3.0.0-pre.8"
  },
  + "prism": {
  + "npm": "prism",
  + "semver": "^4.0.0"
  + },
  "prism-element": {
  "npm": "@polymer/prism-element",
  "semver": "^3.0.0-pre.8"
  