- truncate pixel values to 1dp since there's really no point being told
  the Size is 302.98732587287 by 648.28498579187.

- describe more Widgets so that debugDumpApp() is more useful.

- remove bufferValue from ProgressIndicator (cc @hansmuller) since it's
  not yet implemented.

- half-hearted toString() for ThemeData. There's no point making a
  complete one, since it would cause line-wrap even on big monitors in
  debugDumpApp dumps, and you can easily get the actual values from a
  debugging if that's the issue.

- flesh out BoxConstraints.toString() so that fully unconstrained and
  fully infinite constraints are called out explicitly. I experimented
  with adding even more special cases, e.g. calling out unconstrained
  widths with fixed heights, etc, but it made the output less readable.

- remove a redundant _updateVariable() in AnimatedContainer (cc
  @abarth).

- add more information to RenderView.toString().