add StringBuffer/Builder.reverse

fixes ISSUE 449

Change-Id: I342e7ed86c0e4efa90d09bdfe0d72c79db2654c5
Review-Link: https://gwt-review.googlesource.com/#/c/2431/

Review by: goktug@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@11591 8db76d5a-ed1c-0410-87a9-c151d255dfc7
6 files changed