ES6 symbols: enable symbols as weak map keys

R=mstarzinger@chromium.org
BUG=

Review URL: https://codereview.chromium.org/12456004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13829 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Wait for sweeper threads in EnsureSweeperProgress() only if the main thread finished its sweeping phase.

BUG=

Review URL: https://codereview.chromium.org/12462002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13830 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Make IC patching resilient to flushing of the original target() ic.

Review URL: https://chromiumcodereview.appspot.com/12451003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@13831 ce2b1a6d-e550-0410-aec6-3dcde31c8c00