Allow duplicate property names in classes

ES6 no longer makes duplicate properties an error. However, we
continue to treat duplicate properties in strict mode object
literals as errors. With this change we allow duplicate properties
in class bodies. We continue to flag duplicate constructors as an
error as required by ES6.

BUG=v8:3570
LOG=Y
R=marja@chromium.org

Review URL: https://codereview.chromium.org/677953004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24933 ce2b1a6d-e550-0410-aec6-3dcde31c8c00