Top-level await integration - #4352

The purpose of this patch is to nail down how HTML would
adjust to the needs of top-level await. The relevant change is,
the module.Evaluate() method will return a Promise that resolves
when the module is ready, and this will be passed to
FinishDynamicImport to wait on before resolving the Promise it returns.
It doesn't seem like anyone else needs to wait for a module script
to "finish" evaluating besides dynamic import.

• At least two implementers are interested (and none opposed):
  • Chrome
  • Firefox
  • Safari
• Tests are written and can be reviewed and commented upon at:
  • [blink] Add top-level-await tests web-platform-tests/wpt#25532
• Implementation bugs are filed:
  • Chrome: https://bugs.chromium.org/p/chromium/issues/detail?id=1022182
  • Firefox: https://bugzilla.mozilla.org/show_bug.cgi?id=1519100
  • Safari: https://bugs.webkit.org/show_bug.cgi?id=202484

(See WHATWG Working Mode: Changes for more details.)

/infrastructure.html ( diff )
/references.html ( diff )
/webappapis.html ( diff )