Allow people to add @const to constructors in order to
prevent them from being able to be subclassed. This will
result in an error by default.

R=johnlenz
DELTA=69 (69 added, 0 deleted, 0 changed)


Revision created by MOE tool push_codebase.
MOE_MIGRATION=6052

git-svn-id: http://closure-compiler.googlecode.com/svn/trunk@2428 b0f006be-c8cd-11de-a2e8-8d36a3108c74