title: my angular 7 PWA can't found file 'ngsw.json' when is offline
body: <p>I am doing a PWA aplication and it works well (Pass al the audits of Chorme) but when i want to try it in offline mode. a 504 TIMEOUT error appears. The DevsTools(Chorme) shows 'An unknown error occurred when fetching the script,
  Failed to load resource: net::ERR_INTERNET_DISCONNECTED' and Network slice shows a failed when the app try to get "ngsw.json?ngsw-cache-bust=0.30493506524068925"</p>
  
  <p>This is my package:</p>
  
  <pre><code>{
    "name": "avl",
    "version": "0.0.0",
    "scripts": {
      "ng": "ng",
      "start": "ng serve",
      "build": "ng build",
      "test": "ng test",
      "lint": "ng lint",
      "e2e": "ng e2e"
    },
    "private": true,
    "dependencies": {
      "@angular/animations": "^7.2.8",
      "@angular/cdk": "^7.3.4",
      "@angular/common": "~7.2.0",
      "@angular/compiler": "~7.2.0",
      "@angular/core": "~7.2.0",
      "@angular/flex-layout": "^7.0.0-beta.24",
      "@angular/forms": "~7.2.0",
      "@angular/http": "^7.2.8",
      "@angular/material": "^7.3.4",
      "@angular/platform-browser": "~7.2.0",
      "@angular/platform-browser-dynamic": "~7.2.0",
      "@angular/pwa": "^0.13.8",
      "@angular/router": "~7.2.0",
      "@angular/service-worker": "^7.2.13",
      "@mapbox/leaflet-pip": "^1.1.0",
      "@types/fontfaceobserver": "^0.0.6",
      "@types/leaflet": "^1.4.3",
      "@types/pixi.js": "4.7.2",
      "cerialize": "^0.1.18",
      "core-js": "^2.5.4",
      "fontfaceobserver": "^2.1.0",
      "leaflet-pixi-overlay": "^1.6.0",
      "moment": "^2.24.0",
      "net": "^1.0.2",
      "pixi.js": "4.7.2",
      "rxjs": "~6.3.3",
      "rxjs-compat": "6.3.3",
      "tslib": "^1.9.0",
      "wellknown": "^0.5.0",
      "zone.js": "~0.8.26"
    },
    "devDependencies": {
      "@angular-devkit/build-angular": "^0.13.7",
      "@angular/cli": "~7.3.3",
      "@angular/compiler-cli": "~7.2.0",
      "@angular/language-service": "^7.2.8",
      "@types/jasmine": "~2.8.8",
      "@types/jasminewd2": "~2.0.3",
      "@types/node": "~8.9.4",
      "codelyzer": "~4.5.0",
      "jasmine-core": "~2.99.1",
      "jasmine-spec-reporter": "~4.2.1",
      "karma": "~4.0.0",
      "karma-chrome-launcher": "~2.2.0",
      "karma-coverage-istanbul-reporter": "~2.0.1",
      "karma-jasmine": "~1.1.2",
      "karma-jasmine-html-reporter": "^0.2.2",
      "protractor": "~5.4.0",
      "ts-node": "~7.0.0",
      "tslint": "~5.11.0",
      "typescript": "~3.2.2"
    }
  }
  </code></pre>
  
  <p>this is my ngsw-config.json</p>
  
  <pre><code>{
    "index": "/index.html",
    "assetGroups": [
      {
        "name": "app",
        "installMode": "prefetch",
        "resources": {
          "files": [
            "/favicon.ico",
            "/index.html",
            "/*.css",
            "/*.js",
            "/*.json"
          ]
        }
      }, {
        "name": "assets",
        "installMode": "lazy",
        "updateMode": "prefetch",
        "resources": {
          "files": [
            "/assets/**",
            "/*.(eot|svg|cur|jpg|png|webp|gif|otf|ttf|woff|woff2|ani|json)"
          ]
        }
      }
    ]
  }
  </code></pre>
  
  <p>I expect to see the html of the app with his css</p>