[dart:html] Add script to generate dart:html prototype legacy events

Adds a flag `--generate-prototype-events` to the dart:html generation
scripts, which causes a `prototype_events.dart` file to be generated
with EventStreamProviders and extensions for all events generated
in legacy dart:html. The generated file can be copied and pasted into `html_events.dart` to be used in the new dart:html prototype.

The script `prototype_htmleventgenerator.py` reuses as much common functionality as possible from the `htmleventgenerator.py`.

There were many edge cases to consider, like:
  - de-conflicting names with different event types (e.g. ProgressEvent onError vs SpeechRecognitionErrorEvent onError)
  - hiding deprecated types that don't appear in the Web IDL and haven't been needed in the glue code prototype yet
  - renaming extension on-types that have been renamed from the Web IDL
  - hiding custom events, which then need to be added to the prototype by hand
  - prefixing some events with the correct web library

Change-Id: I6ab944d74ede6d8a2178bbf9aa580a6ab7d67a77
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/259063
Commit-Queue: Riley Porter <rileyporter@google.com>
Reviewed-by: Srujan Gaddam <srujzs@google.com>