Array.concat: properly go to dictionary mode when required

BUG=chromium:387031
LOG=y
R=danno@chromium.org

Review URL: https://codereview.chromium.org/342333002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21903 ce2b1a6d-e550-0410-aec6-3dcde31c8c00