The theme fallback mechanism was busted by the shadowThemeOnly change.

Fixes #6562

Fixes https://github.com/flutter/flutter/issues/6561

* Use dark theme in GridTileBar

Reduce IconTheme, Typography hardcoding by applying a dark theme to the
contents. This is still not ideal; the theme brightness should really be
driven by backgroundColor.

* Add back IconTheme

Since IconTheme.of() finds the closest IconTheme, not the closest
IconTheme or Theme (which specifies IconThemeData), the previous commit
doesn't necessarily override any IconThemes futher up the tree.

* refactor device descriptions method - easier to add columns
* show device SDK version

See https://github.com/flutter/flutter/issues/6539

Soon the engine will send this information through platform messages.

Eliminates hardcoding of Typography.white in favour of picking up the
platform default typography from ThemeData.

Fixes https://github.com/flutter/flutter/issues/6451

If a URL is not explicitly specified by the test author, check for an
environment-specified URL before falling back to the default value.

This should help new users figure out how to set up their constructors
and use config data from the build method of a State.

Mostly around pointing people towards Block more aggressively.