Allows Linkers to mark themselves as shardable by including a
private field with a certain name. Most linkers should use the
@Shardable annotation, but that doesn't work for linkers that
need to also compile against older versions of GWT.

Review at http://gwt-code-reviews.appspot.com/678802

Review by: zundel@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@8372 8db76d5a-ed1c-0410-87a9-c151d255dfc7