* Allow FloatingActionButton to not have a heroTag.
* Allow FloatingActionButton to not have a child.
* Allow Tooltip to not have a child.
* Improve the debug output of the default FloatingActionButton hero tag.
* Improve the error message in the Hero clashing-tag case.
* Improve the debug output of the Hero widget.
* Improve the debug output of gesture-related widgets.
* Minor improvements to documentation.
* Fix some typos in comments.
* Fix some style nits.