docs(quickstart): edit Dart & TS QuickStarts

[kwalrath]

@wardbell & @chalin, here are a few edits resulting from my review of the latest QuickStart. OK by you?

[chalin]

Nice edits.

Btw, rather than define an extra variable named _anAppDir, in the prose use: #{_an} #{_appDir}. The variables _a and _an are defined in public/_includes/_util-fns.jade (L18-21).

Otherwise, LGTM.

[wardbell]

LGTM subject to Patrice's advice.

[kwalrath]

I've made Patrice's suggested edit, plus rearranged a bit to feature dart_to_js_script_rewriter. And a few misc. copy edits. I'm planning to submit, as it'd be hard to judge the changes without actually building the docs.