Chap 6 previous chap made consistent & follow Dart Style Guide

- Chap 6 index.html: some unnecessary differences with chapter 5’s
index.html have been eliminated.
- Added types on public class members to follow the Dart Style Guide.
- Removed whitespace from chap 5 files to eliminate unnecessary
differences with chap 6 files.
- Minor edit to chap 3 & 4 index.html to only show “Recipe Details”
heading when a recipe is being shown. This is more consistent with the
app behavior of chap 6 (which renders the recipe details via a
completely self-contained view).
Merge pull request #75 from chalin/patch-chap6-etc

Chap 6 made consistent with previous chap; and follow Dart Style Guide