Ben says that mImmutable is no longer useful.
For the sicking told me so.
Well.. I'm not sure. I thought the only reason we had that was to enable lazily getting the size. But it sounded like the only code path that I knew of where we would need to lazily get the size now eagerly gets the size. Code inspection should tell you if mImmutable is ever used for anything useful, no?
<http://mxr.mozilla.org/mozilla-central/source/content/base/src/nsDOMFile.cpp#715> seems to be the only place where this does something useful. There's also a bunch of places where we assert its value.