add an example for type erasure

see http://wasabiz.hatenablog.com/entry/2013/08/18/142749 (ja)
update the example of type erasure
extract FusedAssignmentExpression from AssignmentExpression
fix LCSE on fused assignments
Merge branch 'gfx/refactor-assign-expr'
Merge branch 'master' of github.com:jsx/JSX
remove expression transformers (for now)
Merge remote-tracking branch 'origin/master' into refactor-transformer
[refactor] transform statemeents by state-based strategy
make CodeTransformer's methods private
clean up
change the naming convention for label names
remove duplicate test options
rename s/_labelMap/_labelStack/
cleanup, renaming some variables