Reorder categories and fix enum masthead links

[jcollins-g]

This addresses #1323 and #1225.

[devoncarew]

lgtm!

In several places it looks like dartfmt has split a one line if statement; so:

if (foo) bar();

into:

if (foo)
  bar();

which is correct given the line length, but I believe Dart's style prefers to use curlies braces for if statements (unless the whole if statement can fit on one line).

So, instead:

if (foo) {
  bar();
}

https://www.dartlang.org/guides/language/effective-dart/style#do-use-curly-braces-for-all-flow-control-structures

[keertip]

Looks good!