ES6: Duplicate properties are no longer an error

This removes the duplicate property checker and updates the tests.

BUG=v8:3498
LOG=Y
R=marja@chromium.org

Review URL: https://codereview.chromium.org/459463002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23239 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Delete dead GetType from elements accessors

BUG=

Review URL: https://codereview.chromium.org/489033002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23240 ce2b1a6d-e550-0410-aec6-3dcde31c8c00