refractor service worker

[chenxsan]

We have webpack.ssg.js to generate those static html pages, but the current service worker is not taking full advantage of them.

For example, when we open view-source:https://webpack.js.org/guides/, we expect dist/guides/index.html to be returned. Instead /app-shell/index.html is returned at the moment.

This pull request refractors the service worker replacing GenerateSW with InjectManifest which gives us more control.

• add some e2e tests

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[chenxsan]

Got a weird issue in CI https://travis-ci.org/github/webpack/webpack.js.org/jobs/741364115#L283:

setup compilation HtmlWebpackPluginTypeError: Cannot read property 'Compilation' of undefined
    at compiler.hooks.thisCompilation.tap (/home/travis/build/webpack/webpack.js.org/node_modules/html-webpack-plugin/index.js:210:26)
    at Hook.eval [as call] (eval at create (/home/travis/build/webpack/webpack.js.org/node_modules/tapable/lib/HookCodeFactory.js:19:10), <anonymous>:64:1)
    at Hook.CALL_DELEGATE [as _call] (/home/travis/build/webpack/webpack.js.org/node_modules/tapable/lib/Hook.js:14:14)
    at Compiler.newCompilation (/home/travis/build/webpack/webpack.js.org/node_modules/webpack/lib/Compiler.js:915:30)
    at hooks.beforeCompile.callAsync.err (/home/travis/build/webpack/webpack.js.org/node_modules/webpack/lib/Compiler.js:957:29)
    at Hook.eval [as callAsync] (eval at create (/home/travis/build/webpack/webpack.js.org/node_modules/tapable/lib/HookCodeFactory.js:33:10), <anonymous>:40:1)
    at Hook.CALL_ASYNC_DELEGATE [as _callAsync] (/home/travis/build/webpack/webpack.js.org/node_modules/tapable/lib/Hook.js:18:14)
    at Compiler.compile (/home/travis/build/webpack/webpack.js.org/node_modules/webpack/lib/Compiler.js:952:28)
    at compiler.hooks.watchRun.callAsync.err (/home/travis/build/webpack/webpack.js.org/node_modules/webpack/lib/Watching.js:113:19)
    at Hook.eval [as callAsync] (eval at create (/home/travis/build/webpack/webpack.js.org/node_modules/tapable/lib/HookCodeFactory.js:33:10), <anonymous>:26:1)

Downgrading html-webpack-plugin fixed the problem.

[chenxsan]

We're testing against the production site now, I think it makes more sense than testing against development site.

[chenxsan]

Oops, we have a problem GoogleChrome/workbox#2493 now:

Uncaught ReferenceError: regeneratorRuntime is not defined

[montogeek]

Please move this to an external dependency.