Changes default logger to print native exception stacks

Change-Id: Icd69b1403604706539588d18e80af357e5c607eb
Review-Link: https://gwt-review.googlesource.com/#/c/9821/
Stops enumerating the contents of hidden directories, to save time.

GWT's default resource scanning process already takes some care to 
ignore .svn and similar directories, but does so at a stage after "all"
files have been enumerated in all registered directories. As a result 
the GWT compiler is still paying the full cost of that large file list 
enumeration. This patch changes the file enumeration process to never 
even look inside intentionally hidden folders, thus avoiding any 
resulting file enumeration cost.

Change-Id: Ibd202c17d8287c240660e4f1169643c84489fe1a
Review-Link: https://gwt-review.googlesource.com/#/c/9850/