Change the order of groups on dartdoc pages

[Hixie]

The order right now is a bit weird (especially that typedefs come so early on the library page).

Here are the suggested new orders:

• Top-level page: flutter, dart core, flutter_markdown, flutter_test, flutter_driver.
• Library pages: classes, constants, properties, functions, enums, typedefs, exceptions.
• Enum pages: constants, properties, methods, operators.
• Class pages: constructors, properties, methods, operators, static methods, constants.

cc @sethladd

[sethladd]

This list is approved. Go forth and make it awesome.

[sethladd]

cc @astashov if you're looking for one P2, this one is an easy one :)