Global attribute name standardization for widgets
Better attribute names + other cleanups in widgets
TEMP
Merge branch 'master' of https://github.com/dart-lang/spark into umop-markers-zoom
Merge branch 'master' of https://github.com/dart-lang/spark into umop-markers-zoom
#325

- 1), 2) should be fixed with this update
- 3) should be defined what should happen

Open question:

- I don't know if large and small should be exclusive or.
I guess so, but it was not.

Note:

- I tried to minimize updates to the btnClasses map but that didn't work
I filed a but, see code comment on method `updateBtnClassesJob()`

#786

- I think the button tests is thoroughly.
It can be refined of course if issues occur
or if someone has ideas what can be additionally tested

Note:

- drone.io doesn't install content_shell by default
this has to be done manually (I haven't done it myself yet)
if you add `test/run.sh` to the build script for drone.io the test will probably fail
Merge branch 'master' of https://github.com/dart-lang/spark into umop-markers-zoom
TEMP
Improved splitter visual
Implemented scrolling
Cleaned up implementation (scrolling)
Organized a bit for style
Merge remote-tracking branch 'upstream/master' into splitter-css-style
Simplified code
stacking context trick
Temporary commit before merge to get deactivate (overriding activate)
Merge branch 'master' of https://github.com/dart-lang/spark into umop-outline-remember_position
Overriding activate and deactivate to store outline scroll position
Initial setup and implementation of SparkPreferences
Moved CachedPreference and BoolCachedPreference into preferences.dart