This makes several changes to flutter web app bootstrapping.

• The build now produces a flutter_bootstrap.js file.
  • By default, this file does the basic streamlined startup of a flutter app with the service worker settings and no user configuration.
  • The user can also put a flutter_bootstrap.js file in the web subdirectory in the project directory which can have whatever custom bootstrapping logic they'd like to write instead. This file is also templated, and can use any of the tokens that can be used with the index.html (with the exception of {{flutter_bootstrap_js}}, see below).
• Introduced a few new templating tokens for index.html:
  • {{flutter_js}} => inlines the entirety of flutter.js
  • {{flutter_service_worker_version}} => replaced directly by the service worker version. This can be used instead of the script that sets the serviceWorkerVersion local variable that we used to have by default.
  • {{flutter_bootstrap_js}} => inlines the entirety of flutter_bootstrap.js (this token obviously doesn't apply to flutter_bootstrap.js itself).
• Changed IndexHtml to be called WebTemplate instead, since it is used for more than just the index.html now.
• We now emit warnings at build time for certain deprecated flows:
  • Warn on the old service worker version pattern (i.e.(const|var) serviceWorkerVersion = null) and recommends using {{flutter_service_worker_version}} token instead
  • Warn on use of FlutterLoader.loadEntrypoint and recommend using FlutterLoader.load instead
  • Warn on manual loading of flutter_service_worker.js.
• The default index.html on flutter create now uses an async script tag with flutter_bootstrap.js.