Fixes #6591

This makes https://flutter.io/basic-widgets/#layout-models redundant
with the dartdocs.

Fixes #6480

These aren't needed anymore.

This command is no longer needed. Also, remove some stray references to Mojo.

If a recognizer is interrupted by an exception from a callback, it could be
left in an inconsistent state and be unable to process future events

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.