Always regenerate info using all sources if any have changed

Because we don't know if a new or modified file includes changes to the class hierarchy, we regenerate info for all sources any time any one has changed.  An alternative would be to generate info first for the new or modified file and then (potentially) regenerate info for more source files in the class hierarchy, but this makes the generate-info.js task far more complicated.
Move developer guide from wiki to CONTRIBUTING.md
Use goog.dom.classlist functions

The goog.dom.classes functions have now been deprecated.
See google/closure-library@97e8a0c
Merge pull request #2370 from gingerik/classlist

Use goog.dom.classlist functions (2)
Merge pull request #2369 from probins/contribute

Move developer guide from wiki to CONTRIBUTING.md.
Merge pull request #2356 from tschaub/regenerate-info

Always regenerate info using all sources if any have changed.
Remove duplication from contributing.md
Merge pull request #2374 from probins/patch-1

Remove duplication from contributing.md