docs(toh-5/dart): review edits and code cleanup

[chalin]

• Adjust text which used to say "create a new 'styles.css'" when we've been carrying it around since QuickStart.
• The HTTP tutorial chapter is no longer "forthcoming".
• This commit also adds a missing docregion to a toh-4 file that is referrenced in toh-5.
• Fixed prose over indentation (which appeared in over half the file).
• Copyedits.

Code:

• Add missing return types.
• Handle invalid numeric route hero ids.
• Rename HERO mockHeroes as used in earlier tutorial chapters.
• Use Dartisms (cleanup from ts2dart transpilation).

Contributing to #1628.

[chalin]

@kwalrath @thso : prose and code changes are ready for review.

[kwalrath]

If we're editing this, I'd use sentence caps but not add commas at the end. (Periods would be fine, though.) I'd also add a colon after "plan". For example:

Here's our plan:

• Turn ... navigation.
• ...
• Tie ... structure.

[chalin]

That is my preference too. Done.

[kwalrath]

The changes look good, with the exception of what I commented on. (This page needs a good copy edit, but I tried to avoid looking at anything you didn't change!)

[chalin]

I agree about further copy edits. For now, I have addressed the points that you raised, and made the changes to both the TS and Dart sides so that they match.

[kwalrath]

LGTM. Merging now...