Clone frozen chunk pre-modification (since 46119efb doesnt clone).

* Another case where commit 46119efb is biting us. That commit
  started cloning tokens on demand, which means we have to clone
  in other places on demand as well.

* Eliminates crasher: node roundtrip-test.js 'Metaclass'

* No change in parser test results.

Change-Id: Ia9eed0e5c9f8b704b3c557a7896984d053ae6e06