blocks labeled by "break", "continue", and "return".  BreakTargets are
the only jump targets that appear in the AST, the only ones that can
be uninitialized, and the only ones that can be shadowed.
Review URL: http://codereview.chromium.org/42008

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1475 ce2b1a6d-e550-0410-aec6-3dcde31c8c00