681b366abe
Adds CicularProgressIndicator and LinearProgressIndicator Setting a ProgressIndicator's value to null (the default) makes it an "indeterminate progress" or activity indicator. The indeterminate animations for both kinds of progress bars are essentially the same and wrong vis the Material Design spec, http://www.google.com/design/spec/components/progress-activity.html. I'll improve conformity with the visual design in a future CL.